Yeah, sometimes they get 'trapped' in doorways and dont react when you attack them, this is how 3 of us killed the level 70 sim in the org construct 
I dont know how that happens.
Also, then NPCs leave the mission area, they dont react to damage as you aren't on the mission area and dont have anyone to target, this is the same reason you can heal in the Elevator while they stant just on the other side of the elevator and dont attack you.