Listing multidisc albums with different composers

[Uoppi]

The album tag was in this format:
Heinz Holliger Edition [disc 01: 01. Vivaldi: Oboe Concertos]

To be able to use separate cover art for each disc, I also added DISCSUBTITLE:
disc 01: Vivaldi: Oboe Concertos

What I tried next was only using "Heinz Holliger Edition" as the title (i.e., removing the [disc...] parts from the album name completely) and relying on DISCSUBTITLE only (because I don't know if you are supposed to use the [disc 01] and DISCSUBTITLE together or if they will conflict.)

With the above change, the album entries have gone down to 2 from 5. This time however - and quite unexpectedly - the albums are apparently listed based on the ARTIST NAME tag. The disc that has only one ARTIST NAME tag for the whole album is listed separately. The rest of the discs have several ARTIST NAME tags across different tracks and they are shown together. (See attachment)

EDIT: Something weird is going on. Just for testing, I re-inserted the [disc 01] suffixes in the ALBUM tags. Whereas previously it resulted in 5 different entries, I now see the same 2 entries as in the attachment. Somehow it appears that whether there are one or more ALBUM ARTIST and/or COMPOSER tags (who knows, maybe other tags too?), it can screw up the sorting. Discs that have multiple values (different composers/artists on different tracks), get grouped together as they should but those with a single value are listed separately. Could it be a bug because I'm completely lost with the logic?