This regression was found out retrospectively and traced to the RHEL 8.3.0 GA, where 'pam' and 'systemd-pam' packages were updated.
The MariaDB plugin haven´t worked ever since.
MariaDB introduced PAM_v2 plugin in 10.4 release. (So in MDB >= 10.4 there are both plugin v1 and v2 available)
The PAMv2 plugin works fine.