Extra Notes:

The only people that need to log out in order to kill the instance are those with the quest active.

Example:

Party Members: A, B, C, D, E, F, G, and H.
Only A and B have the quest flagged.

1) Instance starts to go downhill, someone gives the order to flee the battle.
2) Party members B, C, D, and E manage to escape.
3) Party members A, F, G, and H are dead within the instance.
4) The only person that needs to log out in order to kill the instance is A, since he's the only one with the quest flagged.

The bug only happens sometimes, so I think it has to do with whether or not the last person to die/leave had the quest flagged or not, but I haven't figure that part out.