We were playing the VexCaliber mission and we completed the second damage phase. When we went into the portal into the third and final damage phase we kept getting joining allies and returned to the second room. We managed to kill the boss in our 5 second intervals before it would take us back to the second room, hoping that when the boss was killed we could finish the mission. However, when the boss was killed we were still in the second room which turned into a turn back zone and wiped us. Not sure if anyone else has seen this bug, we are about to continue on with another attempt and we hope it was a one time thing.