I don't think the auto stop/start function is supposed to start back up again just because you have released the brake. Mine will start the engine up if the car in front moves off soon after I have stopped, but I think that is the adaptive cruise control doing that.

Sensitivity to brake pressure seems to vary from car to car, on my R unless I am extremely light (feather touch) on the brake pedal it will switch off, and often switches off before I've even come to a complete halt. If I wiggle the steering wheel even slightly it will start up again.