I don't agree with your assessment that he took a tick of purple circle damage either. He's clearly well out of the circle when he dies.
I suppose you believe it's a coincidence he just happened to be leaving the purple circle right when he died? Lag happens, visual bugs happen. I think it's far more likely that he simply died to the purple with the debuff on than a catastrophic bug of kephess himself that randomly kills people. Especially since many other people aren't complaining about this.