Since no one else has jumped in… Maybe try the “nuclear” option - “The Plex Dance” It’s the official unofficial way of clearing all of the bad metadata out of your database.
The article is a little out-dated. You can find the “Clean Bundles” command in Plex Web under Settings → Manage–>Troubleshooting. I would probably only move the files with the bad thumbnail images, but that’s up to you.
