Cortex Boss Fight not progressing

Killed last boss, died myself, when returned to arena - no loot, no boss, no portal back to map.

This is intended and not a bug. Certain endgame encounters require you to be in the boss arena when the boss dies and goes through his death animation, then drops loot. If you leave at any time prior, then no loot drops and no completion is given. This was to prevent an old exploit, but has remained in place.

If you die, you need to stay dead on the floor until everything is over and done. Similarly, no leaving for a trade either.

Edit: the only bit that may be a bug is the lack of portal.