Dead Ascended Karthus Becomes Invulnerable

The link is to a video I posted on my blog about the bug. I'll explain the video below.

My team was killing Xerath for the buff when the enemy Karthus showed up and died on top of it, going into his passive phase and sapping away at Rammus and Warwick's health which made them move off the buff. Karthus got the buff instead which I thought was pretty hilarious.

But then it became horrifying.

We realized that Karthus was now invulnerable and just fled away from the vengeful spirit for the rest of the game. I've included the full duration just to give clear examples of when Karthus should've taken enough damage to noticeably lower his HP but was unaffected instead. The bug ended when Karthus teleported and that's when we killed him for our last points and won. I hope knowing how the bug gets "cancelled" may help in fixing this issue.