I've noticed this alot. I've a very simple mission where the player group goes throughout the AO, killing baddies. That's it. Whenever the leader of my group dies, sometimes the mission can't be completed, because the AI wont advance to the next waypoint.
The new leader stands there giving the same command over & over again. "move 100m bla bla bla". The rest of the group stands there... rendering the mission borked.