Originally Posted by
GnomeChewer
I've only observed the bug on Normal, but it seems to happen when the boss loses too much (percent) health before the change, it's never happened to me with low raid DPS. It's entirely possible that it also occurs on HC but never gets pushed fast enough to trigger the bug.
Haven't bothered sharing or making a bugtracker about it because if your group is good enough to push that fast then the whole thing is more of an inconvenience than a game-breaking problem. I could only see this being an issue in a speed run tournament which rarely happens anymore, or for a leaderboard speedkill which isn't being tracked properly either.