> Does it help to search for a bug, if I install a version with
> enable-debug?

Probably.  Please do so.

This has nothing to do with FPM but the fact that the linked libraries are not available. Please configure correctly location of libxml2 and OpenSSL (libcrypto) and possibly some other libs.

I have deployed a debug build on the UAT server and a crash happened one day later

I think the backtrace shows, the process is trying to shutdown?

Howto debug further the issue?

Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000000000a11368 in zend_objects_store_put (object=0x7f5e43ef57e0) at /home/mate/php-7.2.6-debug/Zend/zend_objects_API.c:150

(gdb) bt
#0  0x0000000000a11368 in zend_objects_store_put (object=0x7f5e43ef57e0) at /home/mate/php-7.2.6-debug/Zend/zend_objects_API.c:150
#1  0x0000000000a0a05c in zend_object_std_init (object=0x7f5e43ef57e0, ce=0x7f5e3fe56bc0) at /home/mate/php-7.2.6-debug/Zend/zend_objects.c:36
#2  0x0000000000a0a604 in zend_objects_new (ce=0x7f5e3fe56bc0) at /home/mate/php-7.2.6-debug/Zend/zend_objects.c:163
#3  0x00000000009eaf0a in zend_default_exception_new_ex (class_type=0x7f5e3fe56bc0, skip_top_traces=2) at /home/mate/php-7.2.6-debug/Zend/zend_exceptions.c:210
#4  0x00000000009eb1f8 in zend_error_exception_new (class_type=0x7f5e3fe56bc0) at /home/mate/php-7.2.6-debug/Zend/zend_exceptions.c:250
#5  0x00000000009c431e in _object_and_properties_init (arg=0x7f5e4ba1f5a0, class_type=0x7f5e3fe56bc0, properties=0x0, __zend_filename=0xd72d30 "/home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h", 
    __zend_lineno=3231) at /home/mate/php-7.2.6-debug/Zend/zend_API.c:1332
#6  0x00000000009c4373 in _object_init_ex (arg=0x7f5e4ba1f5a0, class_type=0x7f5e3fe56bc0, __zend_filename=0xd72d30 "/home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h", __zend_lineno=3231)
    at /home/mate/php-7.2.6-debug/Zend/zend_API.c:1340
#7  0x0000000000a29255 in ZEND_NEW_SPEC_CONST_HANDLER () at /home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h:3231
#8  0x0000000000aa93f4 in execute_ex (ex=0x7f5e4ba1f4a0) at /home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h:59929
#9  0x00000000009a3a0f in zend_call_function (fci=0x7ffc9fbcc300, fci_cache=0x7ffc9fbcc210) at /home/mate/php-7.2.6-debug/Zend/zend_execute_API.c:819
#10 0x00000000009a2f9b in _call_user_function_ex (object=0x0, function_name=0x7f5e4ba69540, retval_ptr=0x7ffc9fbcc370, param_count=0, params=0x7f5e4ba69550, no_separation=1)
    at /home/mate/php-7.2.6-debug/Zend/zend_execute_API.c:654
#11 0x00000000007ce866 in user_shutdown_function_call (zv=0x7f5e4baa1c48) at /home/mate/php-7.2.6-debug/ext/standard/basic_functions.c:5023
#12 0x00000000009d4ef5 in zend_hash_apply (ht=0x7f5e4baa39c0, apply_func=0x7ce7a2 <user_shutdown_function_call>) at /home/mate/php-7.2.6-debug/Zend/zend_hash.c:1506
#13 0x00000000007cec3d in php_call_shutdown_functions () at /home/mate/php-7.2.6-debug/ext/standard/basic_functions.c:5107
#14 0x000000000092058c in php_request_shutdown (dummy=0x0) at /home/mate/php-7.2.6-debug/main/main.c:1846
#15 0x0000000000ac15ea in main (argc=3, argv=0x7ffc9fbcd3e8) at /home/mate/php-7.2.6-debug/sapi/fpm/fpm/fpm_main.c:1994


(gdb) bt full
#0  0x0000000000a11368 in zend_objects_store_put (object=0x7f5e43ef57e0) at /home/mate/php-7.2.6-debug/Zend/zend_objects_API.c:150
        handle = 2097152
#1  0x0000000000a0a05c in zend_object_std_init (object=0x7f5e43ef57e0, ce=0x7f5e3fe56bc0) at /home/mate/php-7.2.6-debug/Zend/zend_objects.c:36
No locals.
#2  0x0000000000a0a604 in zend_objects_new (ce=0x7f5e3fe56bc0) at /home/mate/php-7.2.6-debug/Zend/zend_objects.c:163
        object = 0x7f5e43ef57e0
#3  0x00000000009eaf0a in zend_default_exception_new_ex (class_type=0x7f5e3fe56bc0, skip_top_traces=2) at /home/mate/php-7.2.6-debug/Zend/zend_exceptions.c:210
        obj = {value = {lval = 140042919300624, dval = 6,9190395369756367e-310, counted = 0x7f5e48754e10, str = 0x7f5e48754e10, arr = 0x7f5e48754e10, obj = 0x7f5e48754e10, res = 0x7f5e48754e10, 
            ref = 0x7f5e48754e10, ast = 0x7f5e48754e10, zv = 0x7f5e48754e10, ptr = 0x7f5e48754e10, ce = 0x7f5e48754e10, func = 0x7f5e48754e10, ww = {w1 = 1215647248, w2 = 32606}}, u1 = {v = {
              type = 1 '\001', type_flags = 0 '\000', const_flags = 0 '\000', reserved = 0 '\000'}, type_info = 1}, u2 = {next = 32606, cache_slot = 32606, lineno = 32606, num_args = 32606, 
            fe_pos = 32606, fe_iter_idx = 32606, access_flags = 32606, property_guard = 32606, extra = 32606}}
        tmp = {value = {lval = 8718968882884247553, dval = 6,9244686821830323e+274, counted = 0x7900000100000001, str = 0x7900000100000001, arr = 0x7900000100000001, obj = 0x7900000100000001, 
            res = 0x7900000100000001, ref = 0x7900000100000001, ast = 0x7900000100000001, zv = 0x7900000100000001, ptr = 0x7900000100000001, ce = 0x7900000100000001, func = 0x7900000100000001, ww = {
              w1 = 1, w2 = 2030043137}}, u1 = {v = {type = 48 '0', type_flags = 54 '6', const_flags = 162 '\242', reserved = 2 '\002'}, type_info = 44185136}, u2 = {next = 0, cache_slot = 0, lineno = 0, 
            num_args = 0, fe_pos = 0, fe_iter_idx = 0, access_flags = 0, property_guard = 0, extra = 0}}
        object = 0x7f5e4870bef8
        trace = {value = {lval = 0, dval = 0, counted = 0x0, str = 0x0, arr = 0x0, obj = 0x0, res = 0x0, ref = 0x0, ast = 0x0, zv = 0x0, ptr = 0x0, ce = 0x0, func = 0x0, ww = {w1 = 0, w2 = 0}}, u1 = {
            v = {type = 0 '\000', type_flags = 0 '\000', const_flags = 0 '\000', reserved = 0 '\000'}, type_info = 0}, u2 = {next = 0, cache_slot = 0, lineno = 0, num_args = 0, fe_pos = 0, 
            fe_iter_idx = 0, access_flags = 0, property_guard = 0, extra = 0}}
        base_ce = 0x7f5e3fe56bc0
        filename = 0x7f5e487548c0
#4  0x00000000009eb1f8 in zend_error_exception_new (class_type=0x7f5e3fe56bc0) at /home/mate/php-7.2.6-debug/Zend/zend_exceptions.c:250
No locals.
#5  0x00000000009c431e in _object_and_properties_init (arg=0x7f5e4ba1f5a0, class_type=0x7f5e3fe56bc0, properties=0x0, __zend_filename=0xd72d30 "/home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h", 
    __zend_lineno=3231) at /home/mate/php-7.2.6-debug/Zend/zend_API.c:1332
        __z = 0x7f5e4ba1f5a0
#6  0x00000000009c4373 in _object_init_ex (arg=0x7f5e4ba1f5a0, class_type=0x7f5e3fe56bc0, __zend_filename=0xd72d30 "/home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h", __zend_lineno=3231)
    at /home/mate/php-7.2.6-debug/Zend/zend_API.c:1340
No locals.
#7  0x0000000000a29255 in ZEND_NEW_SPEC_CONST_HANDLER () at /home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h:3231
        result = 0x7f5e4ba1f5a0
        constructor = 0xd8d41d2180a6e700
        ce = 0x7f5e3fe56bc0
        call = 0x7f5e4978aeb0
        __PRETTY_FUNCTION__ = "ZEND_NEW_SPEC_CONST_HANDLER"
#8  0x0000000000aa93f4 in execute_ex (ex=0x7f5e4ba1f4a0) at /home/mate/php-7.2.6-debug/Zend/zend_vm_execute.h:59929
        orig_opline = 0x0
        orig_execute_data = 0x0
        orig_execute_data = 0x0
#9  0x00000000009a3a0f in zend_call_function (fci=0x7ffc9fbcc300, fci_cache=0x7ffc9fbcc210) at /home/mate/php-7.2.6-debug/Zend/zend_execute_API.c:819
        call_via_handler = 0
        current_opline_before_exception = 0x0
        i = 0
        call = 0x7f5e4ba1f4a0
        dummy_execute_data = {opline = 0x0, call = 0x0, return_value = 0x0, func = 0x0, This = {value = {lval = 0, dval = 0, counted = 0x0, str = 0x0, arr = 0x0, obj = 0x0, res = 0x0, ref = 0x0, 
              ast = 0x0, zv = 0x0, ptr = 0x0, ce = 0x0, func = 0x0, ww = {w1 = 0, w2 = 0}}, u1 = {v = {type = 0 '\000', type_flags = 0 '\000', const_flags = 0 '\000', reserved = 0 '\000'}, 
              type_info = 0}, u2 = {next = 0, cache_slot = 0, lineno = 0, num_args = 0, fe_pos = 0, fe_iter_idx = 0, access_flags = 0, property_guard = 0, extra = 0}}, prev_execute_data = 0x0, 
          symbol_table = 0x0, run_time_cache = 0x0, literals = 0x0}
        fci_cache_local = {initialized = 1 '\001', function_handler = 0x7f5e4bab09b8, calling_scope = 0x7f5e4baafd40, called_scope = 0x7f5e4baafd40, object = 0x0}
        func = 0x7f5e4bab09b8
        __PRETTY_FUNCTION__ = "zend_call_function"
#10 0x00000000009a2f9b in _call_user_function_ex (object=0x0, function_name=0x7f5e4ba69540, retval_ptr=0x7ffc9fbcc370, param_count=0, params=0x7f5e4ba69550, no_separation=1)
    at /home/mate/php-7.2.6-debug/Zend/zend_execute_API.c:654
        fci = {size = 56, function_name = {value = {lval = 140042919292832, dval = 6,9190395365906608e-310, counted = 0x7f5e48752fa0, str = 0x7f5e48752fa0, arr = 0x7f5e48752fa0, obj = 0x7f5e48752fa0, 
              res = 0x7f5e48752fa0, ref = 0x7f5e48752fa0, ast = 0x7f5e48752fa0, zv = 0x7f5e48752fa0, ptr = 0x7f5e48752fa0, ce = 0x7f5e48752fa0, func = 0x7f5e48752fa0, ww = {w1 = 1215639456, 
---Type <return> to continue, or q <return> to quit---
                w2 = 32606}}, u1 = {v = {type = 6 '\006', type_flags = 0 '\000', const_flags = 0 '\000', reserved = 0 '\000'}, type_info = 6}, u2 = {next = 32764, cache_slot = 32764, lineno = 32764, 
              num_args = 32764, fe_pos = 32764, fe_iter_idx = 32764, access_flags = 32764, property_guard = 32764, extra = 32764}}, retval = 0x7ffc9fbcc370, params = 0x7f5e4ba69550, object = 0x0, 
          no_separation = 1 '\001', param_count = 0}
#11 0x00000000007ce866 in user_shutdown_function_call (zv=0x7f5e4baa1c48) at /home/mate/php-7.2.6-debug/ext/standard/basic_functions.c:5023
        shutdown_function_entry = 0x7f5e4ba69570
        retval = {value = {lval = 140042972865744, dval = 6,9190421834441975e-310, counted = 0x7f5e4ba6a4d0, str = 0x7f5e4ba6a4d0, arr = 0x7f5e4ba6a4d0, obj = 0x7f5e4ba6a4d0, res = 0x7f5e4ba6a4d0, 
            ref = 0x7f5e4ba6a4d0, ast = 0x7f5e4ba6a4d0, zv = 0x7f5e4ba6a4d0, ptr = 0x7f5e4ba6a4d0, ce = 0x7f5e4ba6a4d0, func = 0x7f5e4ba6a4d0, ww = {w1 = 1269212368, w2 = 32606}}, u1 = {v = {
              type = 0 '\000', type_flags = 0 '\000', const_flags = 0 '\000', reserved = 0 '\000'}, type_info = 0}, u2 = {next = 1500, cache_slot = 1500, lineno = 1500, num_args = 1500, fe_pos = 1500, 
            fe_iter_idx = 1500, access_flags = 1500, property_guard = 1500, extra = 1500}}
#12 0x00000000009d4ef5 in zend_hash_apply (ht=0x7f5e4baa39c0, apply_func=0x7ce7a2 <user_shutdown_function_call>) at /home/mate/php-7.2.6-debug/Zend/zend_hash.c:1506
        idx = 0
        p = 0x7f5e4baa1c48
        result = 32606
        __PRETTY_FUNCTION__ = "zend_hash_apply"
#13 0x00000000007cec3d in php_call_shutdown_functions () at /home/mate/php-7.2.6-debug/ext/standard/basic_functions.c:5107
        __orig_bailout = 0x7ffc9fbcc530
        __bailout = {{__jmpbuf = {0, -3391657720383890912, 4461872, 140722988438496, 0, 0, -3391657720430028256, 3393276251474926112}, __mask_was_saved = 0, __saved_mask = {__val = {0, 0, 
                140042972430336, 140042880155648, 12884901889, 2, 4613750086661631489, 0, 18193016, 0, 10112443, 140722988434592, 10119926, 0, 18193016, 140722988434608}}}}
#14 0x000000000092058c in php_request_shutdown (dummy=0x0) at /home/mate/php-7.2.6-debug/main/main.c:1846
        __orig_bailout = 0x7ffc9fbcd090
        __bailout = {{__jmpbuf = {0, -3391657720031569376, 4461872, 140722988438496, 0, 0, -3391657720394376672, 3393276606627224096}, __mask_was_saved = 0, __saved_mask = {__val = {9960506, 0, 64, 
                4294967430, 1268777024, 0, 14072368, 68719476876, 140042972430400, 455266533782, 459561500677, 140042972888768, 140042972868608, 140042972430336, 140042972888768, 140042972888736}}}}
        report_memleaks = 1 '\001'
#15 0x0000000000ac15ea in main (argc=3, argv=0x7ffc9fbcd3e8) at /home/mate/php-7.2.6-debug/sapi/fpm/fpm/fpm_main.c:1994
        primary_script = 0x7f5e4ba021e0 "\240\372pE^\177"
        __orig_bailout = 0x0
        __bailout = {{__jmpbuf = {0, -3391657720939636192, 4461872, 140722988438496, 0, 0, -3391657720042055136, 3393276932330134048}, __mask_was_saved = 0, __saved_mask = {__val = {0, 1, 
                140043140272488, 140043080367256, 140043140203752, 0, 0, 140043140273344, 140722988437840, 6974919656, 140722988437824, 4131212846, 4334078, 4294967295, 140043138079430, 
                140043080389648}}}}
        exit_status = 0
        cgi = 0
        c = -1
        use_extended_info = 0
        file_handle = {handle = {fd = 1376739320, fp = 0x7f5e520f5ff8, stream = {handle = 0x7f5e520f5ff8, isatty = 1398796655, mmap = {len = 140043102358456, pos = 140722988437464, map = 0xff8760ae, 
                buf = 0x3fe1d82 <error: Cannot access memory at address 0x3fe1d82>, old_handle = 0x7ffc0000002e, old_closer = 0x7f5e557f3e14 <do_lookup_x+372>}, reader = 0x7f5e520f5ff8, fsizer = 0x846, 
              closer = 0x7f5e55a004e8}}, filename = 0x7f5e4ba02000 "\340!\240K^\177", opened_path = 0x0, type = ZEND_HANDLE_FILENAME, free_filename = 0 '\000'}
        orig_optind = 1
        orig_optarg = 0x0
        ini_entries_len = 0
        max_requests = 100
        requests = 57
        fcgi_fd = 0
        request = 0x2bba150
        fpm_config = 0x7ffc9fbce8ba ""
        fpm_prefix = 0x0
        fpm_pid = 0x0
        test_conf = 0
        force_daemon = -1
        force_stderr = 0
        php_information = 0
        php_allow_to_run_as_root = 0
        __func__ = "main"

#4  0x00000000009eb1f8 in zend_error_exception_new (class_type=0x7f5e3fe56bc0) at /home/mate/php-7.2.6-debug/Zend/zend_exceptions.c:250

This was a Symfony OutOfMemoryException

Very likely caused by https://github.com/php/php-src/blob/PHP-7.2/Zend/zend_objects_API.c#L144. If the object store reallocation causes a bailout the stored size will be twice as large as it actually is, so if a new object is created during shutdown it's going to write past the allocation. The change of the size variable should be moved until after the erealloc.