If it matters, the client is AVSub. This does not happen with subsonic.

I think I see the problem. In Scanner.__find_album, albumartist is passed in as an argument. This is used to retrieve an Artist object, which is then queried for a list of albums. This means that for tracks in a multiple artist album to be grouped together, the album artist tag must be set, if not Scanner will generate multiple album objects for a single various artists album.
This is different behaviour to subsonic, which will fall back to looking at the directory an audio file is in. Could this be adopted for supysonic as well?

See https://github.com/sindremehus/subsonic/blob/master/subsonic-main/src/main/java/net/sourceforge/subsonic/dao/AlbumDao.java#L67 for subsonic behaviour.

Hello.
As you might have guessed, tracks from a various artist release will only be grouped under the same album if their album artist metadata field is properly set. Following Subsonic's behaviour might introduce some inconsistencies such as compilations where each track is from a different artist would be listed as an album of a random artist in this compilation.
This might be cumbersome but I'd rather have the current behaviour rather than trying to be lenient and having the possibility to produce results that would be incorrect.