The bug

Occasionally, zombie spawners will spawn zombie villagers. Zombie villager spawners, conversely, never spawn regular zombies.

How to reproduce

1. Place a bunch of spawners
2. Use zombie spawn eggs in each of them
3. Set the time to midnight
4. Wait, and kill regular zombies as they spawn
5. Eventually, a zombie villager will spawn from the spawner

Notes

Java used to behave the same way, but it was fixed. Grum informed us that not spawning zombie villagers is the intended behavior now. See MC-106146 for more information.

Zombie spawn eggs do not have this problem. They always spawn regular zombies. In addition, zombie spawners also correctly never spawn husks, even in the desert.