this is the oldest article i can find on plex forums but i think reddit has older ones.
i have deleted and rebuild photo library dozens of times trying different scenarios to see if there is only certain folders or files needed. but it seems you need the entire plex server folder for photos to work temporarily .
like others have said it is plex keeping a record of times/stamps/hash of each photo. if you have wrong info before and it scans it then your done in the future.
strange thing is. when you delete the photo library and add it back. its perfect. so what ever combo scanning method they use works fine.
BUT when you run analyze (sometimes) it messed it up and try’s to update certain photos and then the timeline is screwed up.
the funny thing is. this is a feature for plex pass customers, EVEN though i am, i have it turned off, AND it still has the nerve to send my photos to a third party and in return DISABLE functionality of my photos. All because i paid to be a plex pass customer. 
