This could be more innocent than it sounds. Computer data is never actually “deleted” until it’s overwritten with new 1s and 0s — operating systems simply cut off references to it.
This is emphatically not true on iPhones. If it’s surfacing deleted images, it’s a big deal.
I was going to reply with a long explanation, but since you were so emphatic about it I decided not to. You emphatically know more about it than us I guess?
But you say “on iPhones”, not on the cloud services they connect to.
Just sounds like a cloud sync error to me, boring but it happens. I don’t have any issues but I also have all cloud photo services disabled.
Some people claim they don't have cloud sync enabled and also a lot of the photos were deleted several years ago... on different physical hardware hardware (but somehow carried forward through device transfers).
Putting my developer hat on and reading various reports - this smells like Apple had a really bad data loss bug which they quietly fixed by attempting to automatically recover lost photos from some corner of the database that still might have the data. Such as the thumbnail database or a cache.
Backups people. Make sure you have good backups and for your most precious photos not just digital ones - print them. And send a physical copy to your grandparents as a gift - they'll love it and it will be one more place you can recover that photo of your kid's birthday if you ever need to
And if you don't want something in a photo... don't take the photo.
Backing this stuff up is really difficult. The vast majority of macs don’t have enough storage to keep everything local. If you put it on an external drive then it disables some features.
My solution is to keep an old Mac as a backup device that keeps all the files local and backs them up. But that’s far more than most people are prepared to do.
Probably nothing sinister lol they're archiving everything every iPhone user has ever had on their device even if they don't sync and some bug accidentally pulled back the curtain for a minute.