Summary:
Mobs spawned from a spawner despawn when the player moves more than 54 blocks away from them.

Steps to Reproduce:

1. Place a spawner.
2. Use a spawn egg on the spawner.
3. Wait for the spawner to spawn a mob.
4. Move more than 54 blocks away from the mob.
5. Move back to where the mob was.

Observed Results:
The mob despawns when moving more than 54 blocks away from it.

Expected Results:
The mob should not despawn when moving more than 54 blocks away from it.

Original Description:
I was playing on my let's Play World and i have a xp farm and when i was there there were a lot of Zombies while i left it and when i came back after a while there wasn't even a single zombie left