I was doing heroic strikes alone then started winter's run strike. I did everything like I always do until we got to the fallen ketch area then the door wouldn't open even though every enemy was dead. We jumped off the map hoping the door would open once we killed the enemies again it didn't work. Then my two teammates departed and two more people joined then the door opened. So I don't know if it was a glitch
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If one player stays close to the door during that stage, it should open. Seems to glitch closed if players complete the encounter from too far away from the door.