Skeletons are gone after reloading the world

When I was built the Skeleton farm, I'm going to kill skeletons and collect the XP and items. When I'm saving and reloading the world, skeletons are nothing happened in the game.

Video: https://drive.google.com/file/d/19-JKZ9Lv0sCza8rQc9057UdIslS92167/view?usp=drivesdk