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.