Chicken Jockey Baby Zombie Pigmen take damage from magma blocks

Zombie Pigmen spawned as Chicken Jockeys are vulnerable to magma block damage.

I have observed this from Chicken Jockeys spawning on magma blocks in a XP/Gold farm:

1. First the chickens take damage and are killed.
2. Then the now chicken-less Baby Zombie Pigmen riders take damage from the blocks and are killed, despite this type of mob normally is immune to magma block damage.