Icloud backup

Hello! So I had an iphone 8 which had alot of pictures and video taken from it. It stopped working for some reason so I got an Iphone 12. Signed in Icloud and got the pictures and videos which were on Icloud (there were more on my prev iphone 8 but they werenr backed up so they didnt show up on Icloud. I got my iphone 8 fixed and when I switched it on , it had all those pictures and videos which were taken from it (the ones not in icloud) so I was happy that I have those on the iphone 8. After a day something backed up and my iphone 8 ended up being the same as my iphone 12… (all the extra pictures were gone) im do upset. I tried to restore the phone to a prev date but it doesnt have those pictures anymore. How can the data on the phone just disappear? I hope someone can understand what im trying to tell here