Yes, this should be a bug. When I am in a LFG dungeon, I want to turn in my quests so the easiest route is to teleport out of the dungeon and teleport to the dungeon.
But if the leader leaves the group before I could teleport to the dungeon again, the group is disbanded. Now I have to queue up again and wait for another dungeon, but I would prefer this to be different.
Location shouldn’t be a condition and I should become the leader of a group, regardless of me being there or not.
There are workarounds, like asking players to wait or to walk the entire route back to the start, but this shouldn’t be the case.