Hostile mobs are meant to despawn.
This isn’t a bug. This is how the game works. When you die your stuff drops.
This isn’t a bug or cheat it’s just the game if you don’t want to find structures like that then don’t make a copy. Also achievements should not be turned off for copied worlds because there are lots of bugs in local worlds especially which means people have to copy worlds to make backups.
This also affects 1.16.1