The AI doesn't know you are undead if you are wearing the shapshifting mask and are disguised as one of the three living races. If you are wearing a regular armor helmet, the AI is going to know that you are undead and will do as you described. So, in order to get the benefit of not being "healed", you're going to need to sacrifice an armor slot.
For me, the biggest problem is that when you disguise yourself as one of the living races, you behave like a living being and replace your talents. What I wanted was a way to avoid enemies realizing I'm an Undead while being and behaving like one. I still want to heal with poison and not be "healed" by enemies, that would be the optimal way imo.
