Adding some partial problem info: I expect the problem to be in the following lines in fpm/fpm_main.c:

1234                                 } else if (apache_was_here && env_script_name) {
1235                                     /* Using mod_proxy_fcgi and ProxyPass, apache cannot set PATH_INFO
1236                                      * As we can extract PATH_INFO from PATH_TRANSLATED
1237                                      * it is probably also in SCRIPT_NAME and need to be removed
1238                                      */
1239                                     int snlen = strlen(env_script_name);
1240                                     if (snlen>slen && !strcmp(env_script_name+snlen-slen, path_info)) {
1241                                         FCGI_PUTENV(request, "ORIG_SCRIPT_NAME", orig_script_name);
1242                                         env_script_name[snlen-slen] = 0;
1243 SG(request_info).request_uri = FCGI_PUTENV(request, "SCRIPT_NAME", env_script_name);
1244                                     }
1245                                 }
1246                                 env_path_info = FCGI_PUTENV(request, "PATH_INFO", path_info); 

Here SCRIPT_NAME gets compared to PATH_INFO in the strcmp() call. Now when using mod_proxy_fcgi Apache sends SCRIPT_NAME non-percent-encoded as required by the CGI spec. But PATH_INFO is extracted from SCRIPT_FILENAME which is percent-encoded. That means all is fine as long as the PATH_INFO part does not contain anything that gets encoded. As soon as chars which need encoding are contained, the strcmp() fails and SCRIPT_NAME will not get adjusted.

Furthermore, even if SCRIPT_NAME would be encoded, it would still be important, that the percent-encoding of SCRIPT_FILENAME and SCRIPT_NAME use the same character case for the hex digits, eg. not mixing %5B and %5b.

As a workaround I adjusted SCRIPT_NAME in mod_proxy_fcgi to be consistent with SCRIPT_FILENAME even though that's not CGI-spec compliant using the following directive:

ProxyFCGISetEnvIf "reqenv('SCRIPT_FILENAME') =~ m|.*${APP_DIR}(/.*)|" SCRIPT_NAME "$1"

Here APP_DIR is a Unix env var or Define'd var containing the path to the PHP application (could be your DocumentRoot). One can also use the verbatim path there instead of a variable.

the correct fix would probably be to use a percent-decoded path_info in the strcmp() inside fpm_main.c.

This problem is still existing in the current 7.4.8.