Mask of the Stitched Demon does not remove your ES in certain situations

While transformed, if you swap your helm to Mask of the Stitched Demon, it gives you all the benefits of the helm, but does not recheck if you should or should not have ES still. If equipped when human and then transforming and then taking it off, you do not regain your ES back until you helm swap while in human form again, or change zones. It seemingly forgets to recheck the status of that particular line while transformed.

There are probably more interactions in different situations, because the Demon Form shapeshift has some kind of a snapshot mechanic. For example, if you shapeshift while being slowed/chilled, you retain that status for the whole duration of Demon Form.