Mobs despawn when there are in boats

I putted a panda n a boat, went some blocks away and he despawned.

Steps to Reproduce:

1. find any natural generated mob that can despawn 
2. Put it in a boat and go further than your sim. distance
3. return to where you left the boat

Observed Results:
The mob has left the boat

Expected Results:
The mob should remain to the boat