Cortex map bugged

I ran Cortex, died to DoT on the floor enbetween phases. When i came back to the arena, there is nothing, just empty boss arena and map is not complete and I am locked in the arena.

Not a bug - this is working as intended (apart from the lack of portal).

When you die in this map and other similar endgame encounters, the boss will drop no loot and you will get no completion despite killing the boss if you leave the arena after dying. You need to remain in the arena until the boss death animation completes and then the loot drops and completion is given.