Villagers don't turn into zombies when killed by Zombie Villagers

Steps to reproduce:

1. Set difficulty to hard
2. Build a cage and spawn a Villager and a Zombie;
3. Let the Zombie kill the Villager and observe: the Villager will become a Zombie Villager;
4. Kill the Zombie and spawn another Villager, let the Zombie Villager kill the newly spawned Villager.

Expected results:
The Villager turns into a Zombie Villager.

Observed results:
The Villager dies.