Rift Herald currently bugged?

This happened to me in my last game. I was hitting rift herald in the eye, then when I got him down to about 25% hp, he stopped taking True% damage. Eye was still open but he was only taking our base damage. This just happened again in Trick2g's stream when their jungler was hitting the open eye.

Is this a bug or is it some change that happened that I didnt catch?