Dying to a Corrupted Boss in the Arena of a map Boss Reloads the zone without the boss

I'll explain how it happened

I went to clear my first corrupted nexus.
I kill the map boss at the end.
It spawns the corrupted boss.
It kills me inside the map boss arena.
I press "reload at checkpoint".
The games reloads the arena.
The map boss is dead.
The corrupted boss is nowhere to be found.

As I already completed the map, I can't clear the corruption in this zone, so neither I get the points for doing so.