The fix for
MDEV-18863 added the --defaults-group-suffix option to the mysqld startup call.
This makes mysqld read the full mysqld### section for the given suffix, in addition to the group specific settings already explicitly passed to it in the constructed command line string.
This also makes mysqld see the mysqld_multi specific mysqd=... and ledir=... options though, if given, which it does not understand, and so server startup fails.
This makes any mysqld_multi setup that tries to run multiple instances with different mysqld binary versions impossible to start up.