artist_mbid in https://github.com/metabrainz/listenbrainz-server/blob/e575335f90fa8dd39aac00058d3673cbaf945b0d/webserver/views/api.py#L205 doesn't exist.

The last key ('artist_id') also looks wrong, everything else uses artist_mbids, but maybe it should contain the first artist mbid or something.