Zalmarath, The Colossus become un-targetable during fight.

During the Zalmarath fight, when he does the red projectile attack and moves to his left (the player's right), he became un-targetable for me and my minions. He remained this way until he did the same move later in the fight, where he moved back to his right. He was then targetable again. This was in Act 3 Cruel.