Witchery utilizes a "mutate blocks in-world" mechanic to obtain its crops, Wormwood being no exception. It is made by right-clicking with Mutating Sprig on a fully grown block of wheat. If HO's right click harvest is enabled for wheat (can be selectively disabled and then it will work), mutation will not work regardless of the wheat growth state or player sneaking.

ttCore has fixed it by making sneaking with an item disable right-click-harvest, thus allowing mutation to run.