Mundo Lives as He Pleases [GAMEPLAY]

So I was playing as Irelia top vs Mundo on Summoner's Rift 5v5 Ranked, and we were duking it out for first blood. Well, as I use my Equilibrium Strike to kill him, he disappears from the map and comes back with ~ 100 hp. Needless to say, my kill ended up not registering and he got the first blood with a wicked headbutt. He commented on it saying he disappeared from the map, and let me free farm for a minute, but after that everything was normal. I'm not sure how it happened, but I'll update this if I'm able to reproduce it.

TL;DR - Killed Mundo with Irelia E, he goes to zero hp, disappears, and comes back to kill me.