Ok got it. I adjusted the Album Artist to “John Zorn” on these and did the Plex dance yet again. Sure enough, they are now grouped/sorted the way I want.
Unfortunately, it means I can’t view the performing artist without drilling in a bit which isn’t ideal… but I can live with it.
I still don’t understand (or agree with) why these concepts of Album Artist and Sort Album Artist are linked globally and not respected on a per-file basis. It’s a very strange decision.
Maybe someday the Plex team will finally prioritize proper ID3 metadata support (yes I’ve voted)… but I’m not holding my breath.
Thanks for your help @OttoKerner!