do you have backup save or are you able to recover save from before this happened?
i would backup your current save and then try recover save to see if the photos exist in those then you may make room with photos in them to your library and re add it to your current save and photos should hopefully be there since anything downloaded from gallery/library is supposed to keep them
I'm unsure of why this happens though but i think someone solved it like that recently
also as precaution it might be good to save any pictures etc you have showing as should as room in your library or gallery in case it happens again
