This is simple, and has been brought up before, but there are certain Undaunted pledge quests that specify individual bosses instead of completion of the dungeon.
So, why is this a problem?
The problem is if you lose one or more party members during the run (whether bailing out, disconnecting or vote-kicking) and your group has passed the "point of no return" where one of the specified bosses has already been killed, a replacement player joining the group is basically penalized. Once joining the group, they have two choices:
1. Stay with the party and finish the dungeon without completing their pledge quest, or
2. Leave the group and queue again after the cooldown hoping you can start the dungeon from the beginning.
Those are two poor choices, for both members of the original group and the newly arriving replacements. The original group members may find it difficult or impossible to continue without replacements. If their replacements port in and see that they won't be able to complete their pledge, flip a coin, because those are the chances they'll stay or drop.
The later DLC dungeon pledges are basically "Complete the Dungeon", so why are some of these earlier pledges still stuck with this broken format?
Please address this problem; do something about it instead of just ignoring it. If you really want to make this game more accessible, you might want to have a look at inconsistencies like this that cost people time and frustration.