Hello,
So our group was approximately halfway through Dragonmaw Retreat when one player left.
We used the Group Finder to fill the empty slot, when the replacement player joined, he was automatically given party leadership.
Immediately afterward, the original group members had the countdown to be teleported back to their inns and the dungeon was reset. We lost more than an hour of dungeon progression because of this issue and of course couldn't do it again.
The existing group leader should retain leadership and the replacement player should join the current group and enter the existing Dragonmaw Retreat instance WITHOUT affecting the group’s progress.