hmmm. If this is a bug its been here for awhile at least since version 16. I think its more of a suggestion maybe?
Here is the logic. A track is a mix if all the files with the same [Album] value in the same directory:
Have more than one [Artist] value among them
I think this by design personally. In RJC's photo this is an easy fix by simply enable a pane edit and select out Music (Soulchild) and on track 15 (not shown) "featuring Hollywood". After that Beverley Knight will show up instead of Various Artists.
In electronic music, club/dance stuff and Rap you find this issue all the time. In Jazz they don't even bother most of the time - pretty rare to see a Miles Davis album listing "feat. John Coltrane" on the same track. What I do is add these people to a "soloist" field. (in allmusic guide they are under credits for eg.) If I have two principal artists in the AlbumArtist field I tag it "Artist A & Artist B". In the artist field it would be in
all tracks "Artist A; Artist B"
I can see where this might be really important to know though, that I can understand too -- when it really is to me, I retag the track name => eg. your photo 5 [track] Beautiful Contradiction - feat. Musiq (Soulchild) [Name] Beverley Knight [Artist]
Anyway lots of ways to skin a cat, I'm used to it the way it is. I could live with it if it changes too