Zombies do not react to being attacked by other mobs

When a Zombie is attacked by another mob (Skeleton or Blaze), it does not attack the mob that attacked it, even if it cannot see or reach the player.

What I expected to happen was...:
for the the Zombie to attack the last entity that attacked it.

What actually happened was...:
that the Zombie took the Skeleton's/Blaze's attacks indefinitely without retaliating.

Steps to reproduce:
1. Stand near a Skeleton with a Zombie in between you and the Skeleton.
2. Let the Skeleton shoot his arrow.
3. The Zombie will not attack the Skeleton.