Alright, I got two more tricks up my sleeve but then I'm out of ideas:
Trick 1:
Open the photo in Photos, tap on the Share icon, choose Duplicate
See if the duplicate syncs properly
Trick 2:
Note that if the original would be a Live Photo, the duplicate would no longer be a Live Photo with this trick
Open the photo in Photos, tap on the Share icon, choose Save to Files and save it somewhere you can find it
Open the Files app, find and open the photo, tap on the Share icon, choose Save Image
See if the duplicate syncs properly