Reef Map Boss Bug

Today I ran the Reef Map. When i fought the Boss he did not transition into his immunity/channel phase. I could not damage him either. When i left the map and re-entered, he transitioned normally until the next invulnerability phase. I had to again leave the map and re-enter.

The Boss was not empowered by the league mechanic and he was not Maven-Wittnessed. I had a Divine Vessel on the map for the Paragon. I play a Poison Caustic Arrow Ballista build and I have the "Totems Taunt Enemies around them for 2 seconds when Summoned"-Mod. Maybe the Taunt bugs him?