Named Baby Zombie Pigmen despawn.

I have a cage with 2 Adult Zombie Pigmen, and 1 Baby Zombie Pigman. Twice now, I've come back after running around to find that my named Baby Zombie Pigman has despawned. They haven't died, as no drops are on the ground, they also haven't escaped, because I would've seen them.

The only explanation is, is that they despawned, as my other 2 Adult Zombie Pigmen are completely OK.