I just built a witch farm by BionicSon () in 1.10, then I updated to 1.11 a day after I completed it.
Can someone let me know if they changed the way witch hut spawning works in 1.11 because this farm uses 3 spawning floors, and I haven't gotten a SINGLE witch, but I get plenty of creepers, zombies, skeletons, and spiders (Which clog the farm up.)
I have all the caves nearby lit up, and I would hate to have to move my farm to another witch hut.
Slabs make no difference, you're still perfectly fine there.
From what I can find, there's no change to witch huts, other than the initial witches that are spawned as part of generation don't despawn. The fact that you're getting other mobs in there suggests that the NBT data for the world has become corrupted, and the witch hut structure is no longer in the place it originally was. This should not have happened.
Do you know how to look at the NBT data for your world?