I completed the dungeon during the last day of Season of Defiance, before they changed the text describing the requirements, so I don't know if this was a bug or the intended way. Left at a checkpoint, then came back and finished. The triumph was just for the solo run, not the flawless. Don't know if this was a bug or not, some say it works like that, others not. I reported it some time ago too but went unnoticed due to the season update.