https://3v4l.org/hkZM7

It seems something wrong in hash function setting.

3v4l does not support/allow to change session.use_strict_mode=1

https://3v4l.org/KD6I3

I cannot reproduce problem. Try this phpt file and see if you have problem.
You can run single test as follows from PHP source root.

./run-tests.php ext/session/tests/bug71186.phpt


===== test file - save this code as "ext/session/tests/bug71186.phpt" =====

--TEST--
Bug #71186 session.hash_function - algorithm changes
--SKIPIF--
<?php include('skipif.inc'); ?>
--INI--
session.hash_function=sha512
session.save_handler=files
--FILE--
<?php
ob_start();
ini_set('session.use_strict_mode', 1);

session_start();
$orig = session_id();
session_regenerate_id();
$new = session_id();
var_dump(strlen($orig),strlen($new));
session_commit();

ini_set('session.hash_function','sha1');
session_id('invalid');
session_start();
$orig = session_id();
session_regenerate_id();
$new = session_id();
var_dump(strlen($orig),strlen($new));
?>
--EXPECT--
int(128)
int(128)
int(40)
int(40)

Something wrong in bug system. Please disregard the last comment.

I had to add this to the test script:
session.save_path=/tmp



./run-tests.php -v ext/session/tests/bug71186.phpt 

=====================================================================
PHP         : /usr/local/bin/php 
PHP_SAPI    : cli
PHP_VERSION : 7.0.1
ZEND_VERSION: 3.0.0
PHP_OS      : Linux - Linux waf1 3.19.0-41-generic #46~14.04.2-Ubuntu SMP Tue Dec 8 17:46:10 UTC 2015 x86_64
INI actual  : /usr/local/php7/lib/php.ini
More .INIs  : /usr/local/php7/etc/conf.d/modules.ini  
CWD         : /root/php-7-debian/php-src
Extra dirs  : 
VALGRIND    : Not used
=====================================================================
Running selected tests.

=================
TEST /root/php-7-debian/php-src/ext/session/tests/bug71186.phpt
TEST 1/1 [ext/session/tests/bug71186.phpt]
CONTENT_LENGTH  = 
CONTENT_TYPE    = 
PATH_TRANSLATED = /root/php-7-debian/php-src/ext/session/tests/bug71186.php
QUERY_STRING    = 
REDIRECT_STATUS = 1
REQUEST_METHOD  = GET
SCRIPT_FILENAME = /root/php-7-debian/php-src/ext/session/tests/bug71186.php
HTTP_COOKIE     = 
COMMAND /usr/local/bin/php   -d "output_handler=" -d "open_basedir=" -d "safe_mode=0" -d "disable_functions=" -d "output_buffering=Off" -d "error_reporting=32767" -d "display_errors=1" -d "display_startup_errors=1" -d "log_errors=0" -d "html_errors=0" -d "track_errors=1" -d "report_memleaks=1" -d "report_zend_debug=0" -d "docref_root=" -d "docref_ext=.html" -d "error_prepend_string=" -d "error_append_string=" -d "auto_prepend_file=" -d "auto_append_file=" -d "ignore_repeated_errors=0" -d "precision=14" -d "memory_limit=128M" -d "log_errors_max_len=0" -d "opcache.fast_shutdown=0" -d "opcache.file_update_protection=0" -d "session.auto_start=0" -d "zlib.output_compression=Off" -d "mbstring.func_overload=0" -d "session.hash_function=sha512" -d "session.save_path=/tmp" -d "session.save_handler=files" -f "/root/php-7-debian/php-src/ext/session/tests/bug71186.php"  2>&1
FAIL Bug #71186 session.hash_function - algorithm changes [ext/session/tests/bug71186.phpt] 
=====================================================================
Number of tests :    1                 1
Tests skipped   :    0 (  0.0%) --------
Tests warned    :    0 (  0.0%) (  0.0%)
Tests failed    :    1 (100.0%) (100.0%)
Expected fail   :    0 (  0.0%) (  0.0%)
Tests passed    :    0 (  0.0%) (  0.0%)
---------------------------------------------------------------------
Time taken      :    0 seconds
=====================================================================

=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
Bug #71186 session.hash_function - algorithm changes [ext/session/tests/bug71186.phpt]
=====================================================================

Could you paste the content of 
ext/session/tests/bug71186.log

Your system uses this INI file
INI actual  : /usr/local/php7/lib/php.ini
Could you pate the session section of this INI?

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
If this is not the case and you are able to provide the
information that was requested earlier, please do so and
change the status of the bug back to "Re-Opened". Thank you.

cat bug71186.log 

---- EXPECTED OUTPUT
int(128)
int(128)
int(40)
int(40)
---- ACTUAL OUTPUT
int(103)
int(103)
int(32)
int(32)
---- FAILED

grep -i ^session\. /usr/local/php7/lib/php.ini

session.save_handler = memcached
session.save_path = 'xxx.xxx.xxx.xx1:11211,xxx.xxx.xxx.xx2:11211'
session.use_strict_mode = 0
session.use_cookies = 1
session.use_only_cookies = 1
session.name = ZEBRA
session.auto_start = 0
session.cookie_lifetime = 0
session.cookie_path = /
session.cookie_domain =
session.cookie_httponly =
session.serialize_handler = php 
session.gc_probability = 1
session.gc_divisor = 1000
session.gc_maxlifetime = 1440
session.referer_check =
session.cache_limiter = nocache
session.cache_expire = 180
session.use_trans_sid = 0
session.hash_function = sha512
session.hash_bits_per_character = 5
session.upload_progress.name = "ZEBRA_SESSION_UPLOAD_PROGRESS"

The reason why the test fails is

session.hash_bits_per_character = 5

If you change this to

session.hash_bits_per_character = 4

or set this value in the phpt, then it would pass.
According to the test result, it seems it's working as expected. 

I guess you are experiencing this in web environment. Could you execute php script part of the phpt and check the output? What is your Server API? (see phpinfo() output for used Server API)

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
If this is not the case and you are able to provide the
information that was requested earlier, please do so and
change the status of the bug back to "Re-Opened". Thank you.