[DaveBlake]

Do you have "Prefer online information" setting enabled?

Yes, afetr disabling "Prefer online information" import is fine.

Ah ha! So scraper related not tagging.
"Prefer online info" is my most hated setting (if I can have such a thing!). I have only recently discovered that the original idea of that setting was for fully Musicbrianz Id tagged music to be kept in sync with any changes at Musicbrainz without having to re-tag the music files. Well the implemetation had flaws, and then got changed as the idea was lost, then it seemed the idea was for scraping to sort out messy tagging (it can't), and the result is that it can do some unexpected things sometimes.

I am looking to restore the originally intended use for v18, I'll check this what happens to disc sets as part of that. Meanwhile I would recommend "Prefer online info" is disabled.

Also having "ALBUM ARTIST" and "ALBUMARTIST" tags is a little odd. Kodi will process both, but the second one it finds will overwrite the first so that should not be the issue.

It is a relic from the beginning of my digital music archive (~20 years ago). The tags are identical in nearly 100% of the files.

Fine. But if you ever have ALBUM ARTIST = "xxxx", ALBUMARTIST = "yyyy" then Kodi will see this has having 2 album artists "xxxx" and "yyyy", so just be aware of this possible effect.