I have been dealing with this issue for the last 2 iPhones. I have spent no less than 250 hours on trying to find a solution. I am now, again, working with a Sr Advisor and we have engineering working on this problem. It has since been sent up the engineering chain, which means that I will never get a solution. Every time I do a hard reset my photos disappear then after scrolling, closing the app reopening it, etc some begin to repopulate. I have now done 12 over-the-air restores, 3 itune/finder restores with a fresh os, and a peer-to-peer using my 13 pro max, which actually worked until I had another issue a few days later after I sent the phone back. I have signed into the cloud, signed out, reset network settings, turned on and off messages in iCloud and in messages, signed in and out of messages, and turned off messages in Siri/Search and Facetime. The last suggestion redownloads/uploads but after it's all done it goes back to a sporadic showing of the photos in a thread. there is zero stability. Another issue that drives me crazy is I can't scroll to the oldest message thread unless I search by name for an old conversation then the scroll allows me to see older conversations/threads. I can safely say I have tried every single possible solution except restoring to new which I refuse to do. I am happy to collaborate with anyone as I think I am unfortunately in the top .0001% of the most knowledgeable on this subject, but still without a solution. This sucks!