Q:
Player leaving dungeon resets progress for the group?
My guess, and it is a just a guess, is that the person who quit was the “sponsor”, so to speak. As in they entered the dungeon first and activated it. I’ve had this happen once.
However, I still have no figured out how to replace dropped players. Sometimes a player will go permanently linkdead, or will otherwise vanish and we can’t add a new player to the group and have them enter our instance. They always end up in an instance on their own. End result: we have to start over if anyone can’t/doesn’t finish.