Witches do not spawn inside of witch hut bounding boxes. In place of them, are regular hostile mobs. I noticed this from my witch farm, where I used to get exclusively drops from witches, including things like glowstone and gunpowder, I now get primarily regular hostile mob drops such as bones and rotten flesh, see images attached. The light UI was taken in 1.14, whereas the dark UI was taken in the pre-release. 

If you're unable to recreate on a new world, it may be possible that previous witch hut bounding boxes generated in 1.14-1.15 are not working. Thanks!