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.
The original poster says that he died to "Arcing Slash." I am assuming he looked at logs to acquire that information. If the logs truly do say he died to "Arcing Slash", then I'm inclined to believe them. You are certainly right that it could just be visual lag, but I don't think the poster would have mentioned a specific attack unless he checked logs.
Anyway, my original point remains that it is best to use 3 tanks on 16-man so that you guarantee that the Breath tank can be far away and that the taunting tank will never have Touch of the Masters. This is true regardless of what he died to.