The new mob despawning rules are supposed to not despawn mobs with persistence. One of the things that is supposed to cause persistence is the mob hitting the player, or the player hitting the mob. When either of these actions occur, and you move 54 blocks away from the mob, it will instantly despawn.

Steps to Reproduce:

1. Find a naturally spawned mob.
2. Hit the mob or have it hit you.
3. Move 54 blocks away from the mob.