hmm sorry that doesn't help I see the artist field but not the albumartist field in the tag window. Make it visible by clicking in the tab window top left hand corner; also show ALBUM ARTIST.
Pretty sure its empty. Might have something to do with how you upgraded; normally you would leave version 19 in place upgrade to twenty and everything is ported automatically. First check if this data is missing. If this is the case there's a couple things we can try. If not, with version 19 installed, probably should remove version 20 and resintall the upgrade.
If the albumartist field is blank, I would first make a backup of the existing library File=>Library=>Backup
Then restore your last functioning backup from when this worked right (last one from 19?). I'm not talking about the physical backup of the files here (I assume you have that though just in case, right?
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You probably should run a manual autoimport making sure the fix external changes option (protect drives) is checked for anything added to the library after you upgraded. I would also make sure that MC is writing the tag field to the tag embedded in the file for Album artist and any other critical field for that matter. If it has been configured to do this and the library restore does'nt work to your liking, we can do something else by writing back the tags in the files to the library.
If you have just done the upgrade and still have 19 on your PC a reinstall might be simpler. I'd like to make sure you are handling the album artist field correctly (if you've ported over from itunes to version 19 this could be a good thing to straighten out regardless)