I only now figured this out. Since the addition of the "wide hat" model piece for farmers, witches can render that, but it is just transparent by default.

If you add a texture to the area, the witch will render it, but pretty offset compared to villagers.
This is not the first change on the villager model that affected witches, mainly the hat layer rendered in 18w50a, but that model piece was removed, rather than made transparent.

Suggested fix: Leave in the model piece and mapping, but fix the offset instead, no need to limit resource pack makers.