Its a bug hard to see in a screenshot, but happen when the player is walking around the world and a invisible barrier stop you. mobs trap in the other side of the barrier is affected and they cant move. if you cross the barrier with a enderpearl, player cant move inside the barrier.

This bug is related to another reported bug: world spawn in max height and die. if you close world inside that barrier and open again, you spawn at original spawnpoint and die.