I had one prop do that when I first got my H just as I started the motors, I think this was due to being in a rush to get in the air in this occasion I did not double check each prop was securely fastened, I've now got in the habit of holding each motor in turn and try and the same time, twist each prop clockwise & anti clockwise to check they are securely locked in place, not had one fly off since hope this helps you out.
Sorry ignore what I said I mistook you was referring to the Typhoon H props flying off, but as I have the Q500 4k also it maybe you did not tighten the two props quite enough down onto the rubber o ring.