Photos no longer displaying thumbnails properly

plex dance a dozen of those problem photos. Are you familiar with that procedure?
Do they still exhibit the problem after you dance them?

If that didn’t fix the metadata associated with each photo, enable server debug logging, reboot it, and download the logs after a few minutes. Zip and upload Plex Media Server.log into your reply so that folks can check it for obvious issues.

If possible describe the exact steps to reproduce this on a test PMS system with random photos from the Internet (or upload a few).

Do you have a two Plex Media Servers, one on Windows and one on a Mac as indicated by your tags? Do they share the same media but keep separate databases?