That is exactly why I said it's a bug and clearly not as design or at least it is a design flaw.
Now as I mentioned I am not entirely sure from what I understand from decaying touch, I would think it only affect spells that are actually healing spells. So healing from element would not be affected just like necromancy healing effect aren't.
So either they change it to make it that the enemy always get damaged or they just prevent the healing in these situation.
