When you start an Echoing Nightmare in a party, and one party members leaves the game, then there’s no reward chest after completing the Echoing Nightmare. I do not know if it happens only in a party of two (i.e. do you have to be left alone in a party of three or four).
I had that happen once about a few weeks ago. I don’t bother doing any of these topics. Because most of the time you have somebody telling you it isn’t a BUG!!! But you have prove that it does happen.
Because to me it is another glitch. In this game for these things like this to happen and I just live with it. When stuff like this happens I just move on. Now we see if anything is done about it.
It must be a bug. But not one that seems to be adressed as it has been mentioned before in this forum.
And Davey; no reason to remind us how unfair you often feel you are treated in these forums. Cheer up 
Bug triggers every single time when someone leaves after starting the Nightmare, all it takes is to votekick some person.
No reward also means the portal will not close obv.