The PAM authenticator retrieves users from the MariaDB server side in a similar way as the MySQL authenticator, retrieving all candidate users from the MariaDB side mysql.user table, just restricting the related query to users with "WHERE plugin = 'pam'".

So I'd expect it to return a list of users found this way on "show dbusers", but it just returns an empty list instead.

Looking at the code I can see that MyQLAuth implements its own mysql_auth_diagnostics() and mysql_auth_diagnostic_json() functions, while PAMAuth just uses the default users_default_diagnostic() and users_default_diagnostic_json() implementations.