Witches can not spawn in huts generated in a superflat world.

1 witch might be generated with the hut as it is created, but no more will spawn inside the 7x5x9 boundery spawn box of the hut.

To show the bug, generate the world with the settings below.

Game Mode: Survival or Creative
Seed: -692520701
Superflat world preset: 2;2x4,60x3,44;6;biome_1,village
Generate Structures: ON
Allow Cheats: ON (just for the tp command)
Position of hut: /tp 260 64 288

Put half slabs on top of the roof so no mobs will spawn there and choke the mob cap.

Now ordinary mobs (zombies, skeletons, creapers etc.) will spawn in the hut at night but no witches.

In a world created without the superflat setting, only witches will spawn in huts, and no ordinary mobs.

Contact me for more information if needed.