I'm not sure the P3's active breaking is done by slowing down the motors, at least not any more than by pitching the AC against the forward movement vector. It's exactly the opposite of accelerating forward, or "accelerating backwards" so to speak. If that's true then in fact the motors are sped up, not slowed down... Well at least not to the point of unscrewing the props.
I'm saying so because that's how it sounds to me when the P3 is breaking. My other drones make the same rev up noise when breaking, but it´s hard to tell for sure what´s going on. I could be wrong about that of course, but "breaking the aircraft" is not the same as "breaking the motors", so maybe this unscrewing has to do with something else. Maybe.
Anyway, I use the metal hub props (DJI) a lot, I really dig them, and you'd think those are easier to unscrew accidentally... but it's never happened, not even during hard breaking. I give them all 4 a final tightening before taking off, every time. Just to be sure.