Brand new iPhone 15plus - transferred over from an iPhone 11 that was having the same problem..
When you reboot the phone, videos will play normally out of iPhotos and iMessage
Somewhere between a few hours and a day later, videos will no longer play in iPhoto or iMessage. Although in iPhotos, when you click edit you can get videos to play. Otherwise they won’t play.
Rebooting the phone fixes the problem for a while until the next time they randomly stop playing.
I’m out of ideas.