What I expected to happen was
that the baby zombie would despawn and the chicken would stay
What actually happened was
the chicken despawned too
Steps to Reproduce:
1. Spawn a bunch of zombies until you get a chicken jockey.
2. Get far enough so that any hostile mob would despawned.
3. Get back and see that the chicken despawned too.
Um, yeah, no. This was just fixed because the chickens previously weren't despawning and thus you wound up with tons underground, laying eggs which were picked up by other zombies and thus causing them to never despawn. Thus the chicken despawns shortly after the baby zombie.