This is happening to me and I've seen it happen to others as well and it needs to stop.
If you are in the CP levels and group for a normal dungeon and someone in the group say, "I have the quest." More times than not, that person is not in the CP levels yet and their first time in that dungeon with that character.
Stop leaving the group when the quest is done so the people in the group and actually finish the quest. Killing the last boss doesn't end the quest a groupmate is one. They do have to talk to a NPC at some point to finish the quest and some of them do show up slowly especially in a dungeon like Volenfell.
When people leave a dungeon group, the person who has the quest does not have time to complete it because you get kicked out before the NPC shows up to talk to you.
At this point, you've screwed them out of their quest XP, their Undaunted XP, because those are tied to achievements and there's an achievement for completing the dungeon quest.
So, be a good groupmate and make sure when someone says, "Hey, I have the quest." You stay in the group so they can complete it.