Vault of the Crucible said it only needed 3 barrier wardens killed but the barrier to next area was still up. Turns out there was a 4th barrier warden hiding in a corner (was not marked with skull) that had to be killed before the barrier finally dissipated.
Objective even while 4th was still alive was “Travel to the Besieged Vault.”
can confirm this issue. For me killing the 4th dude still didn’t open the barrier to progress. I am stuck with progression now 
Same here, stuick with only finding two barrier wardens, relogged new dungeon so could progress past tier 1.
Workaround that worked for me: Walk back to the entrance of this level and move back up to the previous then come back, don’t have to rekill the entire floor and the last gate opened for me.
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I encountered the same issue.
I confirm that Phaedra’s workaround works. Thank you for sharing it with us.
@CS this is clearly a bug, can you file a ticket for this?
I just got this same bug, the exit is cut off by a fire trap when I already killed all the wardens.
How is this still a bug?
I had that issue, while looking at this post I got killed right beside that barrier by the rotating machine, then i respawed right on the other side of it and was able to continue