I was importing this FreeDB entry: http://musicbrainz.org/freedb/soundtrack/c909900f
I set the release artist credit to Manuelle Campos & Marie de Gélis (2 distinct artists with " & " as join phrase).
=> release and release-group artist credits have been correctly created: http://musicbrainz.org/edit/14929356
=> add medium however is completely fucked up: http://musicbrainz.org/edit/14929358
Now I need to fix each track artist credit manually (Edit #14929396), then all recordings artist credit manually => 30 boring manual actions.
I'm seriously dedicated to this project, so I'll do the extraneous work, but how many beginners/newcomers will just grumble, give up and stay away from MB for a while?