I have located in multiple worlds (existing worlds and freshly created worlds) a few of the Swamp Hut structures. However, I have yet to find a witch. I have tried just about everything from waiting days, changing distances from the Hut and more waiting. I have so far been unable to find even one witch that spawned near or in the swamp hut. If this is not a bug and rather it is a issue with spawning conditions, information and specifics on spawning conditions need to be made known.