Black Barya boss dissapeared between phases

Reference number Bug Report ID: #1,435,358,081

I DPS'd him out of phase 1, and as he was invulnerable and transitioning to phase 2, I started running around the outside ring of the arena. When he was out of my view, he went into the middle hole and never returned. The coward.

This is a 2.6 div invite at this time, please work on the bug so investments like this don't go to waste. Thanks!