For the record I dont really like how in Original Sin 2, poison does heal undead. Undead look like skeletons. So how would poison doing anything to them ?
It's a throwback to classic RPG's (Especially JRPG's) where this was the case. Poison (Bio if you're familiar with Final Fantasy) would heal undead and healing effects would harm them.
It doesn't make a whole lot of sense on the face of it (Also, in various cases "Poison" is being considered "Acid" too, with it being effective against armour and causing degredation... Old RPG's were really bad about this sort of thing).
I believe the general logic leap is that "Poison" is just a term used in lieu of something like "Nercrosis", "Ichor" or "Miasma", some sort of "Unlife" energy that saps away the life force of the living rather than actual poison (Also, by definition, it would be a Toxin not a Poison since you don't make things ingest it...)
Hence it works on skellingtons, ghosts and other things that lack the systems that are necessary for "Poison" to cause harm.
Basically, a long standing poor wording choice.
