Since updating my iPhone 12 to iOS 14.6 I have to restart my phone multiple times a day so that new photos, screenshots or downloaded media will show up in my photos, or the camera roll in any other apps.
I’ve also noticed issues where viewing old photos it gives me some low res version if I zoom in.
If I restart I get the full res version again and any photos I took that hadn't appeared in the camera roll are suddenly there.
My Phone has 64GB of space remaining out of 128GB and is only 3 months old with about 600 photos.
Other people are seeing this same issue - https://discussions.apple.com/thread/252893991