I play as a beastmaster, and died while trying to kill the boss. My minions killed the boss afterwards and the ‘open a portal’ window apeared. When I respawned into the echo of a world I could click the reward chest. The ‘open a portal’ window was still on my screen, a portal was opened after clicking on it. I could use the portal to port back to the echo of a world (while I was already there). Upon entering there was a new reward chest which I also could loot.
I’ll try to reproduce it and post screenshots.