I just ran it for a third time, this time on a different character. At least two people did not get the achievement either. I sent in a support ticket earlier. Hopefully the problem gets fixed. This is really irritating since it blocks all progress for the achievement. 
Showing Posts For Gorshred.5810:
I’m trying to complete my "Dungeon Master" achievement, and I’ve been progressively going through each path that I’m missing from each dungeon. From the "Sanctuary Savior" achievement, I have everything completed except "Explorable Mode-Plunderer’s Path", which I just got finished running. I did not get credit for it. The achievement progress still says I havn’t completed the dungeon. Is the achievement bugged, or am I missing something?