Map Boss Death Not Acknowledged by Game

Hi,

I just killed the boss mob on Stronghold map (the boss with ballista mechanic). The game, however, has not acknowledged the kill. I collected all loot the boss dropped but the arena barrier won't go down, so I am stuck there. The only thing I can do is portal back to town and abandon the map. Consequently, I can not complete any additional content, such as the Vaal temple which is waiting for me, or the remaining Ritual altars.

Please fix.