Swamp huts not spawning witches

Swamp huts spawn one witch on world generation and then do not spawn any more.

To reproduce:

1. Create a new world.
2. Find a swamp hut.
3. Watch over time to see if witches spawn inside -> they do not.

Also affects swamp huts in old worlds. Anyone with a witch farm built around a swamp hut will be able to reproduce this bug easily (no witch drops).