Vexes are hurt by Splash Potions of Harming although they are undead?

What I expected to happen: I expected the Vexes to be immune to Harming, and hurt by Splash potions of Healing.

What actually happened: The Vexes got hurt by Splash Potions of Harming.

Steps to reproduce: Spawn a lot of Vexes, and start throwing Splash Potions of Harming, once it hits one it'll damage the vex

I thought Vexes were undead creatures, thats why I expected the undead rule to apply to them.