A Crown of Stone bug

I am playing a witch summoner and when I got to the Zalmarath fight (quest A Crown of Stone) if he is standing parallel to the stairs that I can to the platform my minions can't damage him.

If he moves from that location to any other my minions start attacking him and he takes damage.

I figured I should report this as a bug, I don't have a recording because I forgot to turn on my recording, this caught me off guard and I was more surprised then I was thinking about reporting it.

Just now when I finished it hit me that I should have recorded the fight.