Bought a Wisp to heal me and it killed me? - Double up

So I was playing double up with my friend and we were on 1 player health so i bought the wisp that gives you gold based on missing player health and then heals you 3 HP. It then insta killed us and knocked us down to -2 HP. Does anyone know if this is intended or a bug of some sort, or if you just try and heal when you are executable if it will always kill you?

Everyone in the lobby was low health and we were probably going to win as I had a 3 star Ahri, kinda annoyed I didnt know about this interaction if its meant to be this way.