Cannot properly see pictures or videos

I‘ve made this post already but I thought the title was a bit too vague and I can’t edit or delete it anymore so I‘ll just have to post this a second time.

I understand that images saved on the iCloud and not locally often can’t be properly displayed and will in those cases have an exclamation mark icon in the bottom right corner to confirm this. However, whereas originally this only happened to me due to poor internet connection, for a couple of weeks this has been happening very frequently and independently of the internet connection. I started getting very random authorisation requests about a month ago and therefore figured I‘d have to properly re-authorize my iPhone to my iCloud account to fix this gallery issue (even though it has consistently been logged into my apple id anyways), so I started using iCloud Keychain since this mandatorily involves authorising my iPhone and shortly after I did in fact start being able to see images and videos on my gallery clearly again but that wouldn’t last longer than a couple of minutes. The situation hasn’t changed since then so I‘m still only ever able to see my photos or videos clearly for a very short duration after de- and reactivating iCloud Keychain and sometimes even that only works after restarting the phone. I don’t see any reason why this should happen since I can load and see any photo and video from my iCloud on Safari on the very same iPhone and I still have spare iCloud space, too. 

Thanks for any help in advance.