try what myself?
you said "Now if all tracks have different artists from album artist, Disc 1 (sub-header) shows two rows, Disc 2 shows one row, Disc 3 shows two rows"
And I say that if any one or more track artist in the expanded panel differs from the album artist then all rows in the expanded panel (so that should be for disc 1,2,3) should show 2 rows ie. show the artist in all cases. I am assuming your discs are on the same album picture and your expanded panel shows more than one column (otherwise see my last bullet point)
I have tested that extensively so i need to see a screenshot to see what is going on.