Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - Suggestions and Feedback Undead running into poison when decayed.

Undead turn poison damage into healing. Decay turns healing damage into physical damage.
That's why it hurts physical armor first.

And yeah, the AI in this game can use some surface pathing improvement. They will either completely refuse to follow you or run to their doom because of the wacky priority given to surface. There's is no "I have to cross it so maybe I should", just "Do it or don't".