Mana on Kill not working with Monsters Gain 25% ES mod

For some reason the Mana on Kill from my rings stop working. There's nothing in the waystones or tablets that should affect that. Playing Pathfinder with Gas Arrow only.

https://youtu.be/oSfGiYqgXjY

*EDIT* I found it! It's the mod "Monsters gain 25% of Maximum Life as ES". It seems the ES on monsters makes it appear as though I don't actually kill them.