Couldn't figure out why core didn't work on my wiki with LDAP login. After some debugging I found out that lgdomain doesn't seem to be set anywhere at all. See https://www.mediawiki.org/w/api.php?action=help&modules=login|query+info
If I just hardcode login_request["lgdomain"] = "mydomain" in api.py, it magically works. The login logic should look in the family file if self.ldapDomain is set and if that's the case include it in the request. Compat doesn't have this problem.