Your photos get cached. As a last resort you could nuke them.
It’s possible you can go to your Photo’s library 3 dots → Refresh All Metadata.
Most users would Plex Dance whatever media isn’t fitting well into PMS anymore.
- Locate a few photos with missing thumbnails in your filesystem and move them to a different directory that Plex never sees.
- Do a scan library files, empty plex trash, clean bundles, optimize database.
- Move the photos back into place just like you would add any new photos to PMS.
- Scan Library Files.
One of those should do it I hope 
