As reported in the forum: https://mahara.org/interaction/forum/topic.php?id=6042
If you set $cfg->usersuniquebyusername, then when a user roams across from XMLRPC, we're supposed to identify their Mahara account by finding the usr.username value that matches their username in the remote system.
This is in contrast to the normal method, in which we look up their remote username in the auth_remote_user table, and use that to map them to a Mahara user.
The problem is that many of the XMLRPC web services methods, used by the Moodle plugins, call a method api/xmlrpc/lib.php : find_remote_user($username, $wwwroot);. And that method calls User->find_by_instanceid_username(), a method which relies on the auth_remote_user table.
What we should do is, if $cfg->usersuniquebyusername is set, find_remote_user() should call User->find_by_username(), the same as what goes on auth/xmlrpc/lib.php : AuthXmlrpc->request_user_authorise()
Marked low priority because it will only rarely cause any problems. Normally, even if you have $cfg->usersuniq
So, this bug will only cause problems if the auth_remote_user record is missing for some reason, or if it has a value different than the user's Mahara username.