Mobs are despawning with name tags on

Mobs despawn with name tags on.

I play a world with two of my friends and we've name tagged a couple of mobs in order to prevent them to despawn. In our last couple of playing sessions all of our mobs that we've name tagged have despawned. They were within chunk borders the last time I checked with a chunk border texture pack. I don’t know why they are disappearing.