It's possible that if you have washout you don't have a full symetrical wing. Therefore inverted you are not making as much lift as you do upright, hence your stability. In harriers and elevators, stall is king. You want the motor to provide lift as well as thrust, in many planes with lets say 4 strokes with bigger props, you get a little lift from the air forced over the airfoil at the center of the wing. On a tapered wing airplane this is the meat of the airfoil and this provides just enough lift to help stabilize the wing in a harrier. You just dont want to go too heavy on wing loading. But get a real thick airfoil and a light wingloading and sometimes these planes are not stable. Moving the cg back helps.