i do not understand respawn on uber bosses

hello, my friend and me were doing the king of the mist, we did a few tries until i killed him, my friend was down, it was his portal.

i could not take the utility belt, so he respawned, but the zone got reloaded without the boss.

result : we got no loot, the portal is closed and the invite wasted.

it this supposed to work this way ? (if it is, i can't see the point of playing in a group. I'm even tempted to say that everything in endgame is made to encourage solo play. I fint that quite weird. )

any advice ?