I was doing some file cleanup, and moved a handful of files from one folder to another (in my file explorer). Then I ran an auto-file rename on them, and realized that two of the files I'd just moved had had their tags swapped. Checking the files themselves showed that their data was unchanged, it was just within MM's database that the info was switched. Of course, I could've just retagged them, but since the files themselves were unaffected, surely I could find a simpler fix?
First I tried moving them back to their original folder; they stayed mistagged. Then I moved the files somewhere outside MM's scanned locations, to remove them from the library, and restarted MM. Then I put them back in their original location. This worked, and the original tags were restored! So I moved them back to the target folder - and their tags got swapped again... So I repeated the above process, removing and re-adding them to the original folder. Finally, I moved them one at a time to the target folder instead of together. This finally worked, and their tags stayed correct.
So there was a happy ending
But I do wonder what caused it, and of course, if it happened once it could happen again.