The LocalCoverArtSearch saving logic is wrong:

if u_entry:
    u_artist, u_album = [db.entry_get (u_entry, x) for x in [rhythmdb.PROP_ARTIST, rhythmdb.PROP_ALBUM]]
    if (artist, album) != (u_artist, u_album):
        print "Not saving local art; encountered media with different artist/album (%s, %s, %s)" % (uri, u_artist, u_album)

This does the wrong thing on albums with multiple artists; it should only check that the album is the same.