I’m curious- did you actually try the complete “Plex Dance” I mentioned previously for the library.
Plex will stubbornly hold onto bad metadata and posters, when you remove items from your libraries (any kind, not just photos) Although it will remove the metadata eventually as part of scheduled tasks, that could be a week depending on when you removed the items.
The only real way to get rid of it immediately is with the dance, in the exact order, and completely. Particularly the “Clean Bundles” step to clear out the metadata.
