Yes, the one-punch knockdowns should happen even when energy of the boxer is high. It basically should happen rarely though, but still should be possible, so that a well placed punch can bring down even a stronger opponent and add a bit of suspence to all the fights!
if you already knew you would win all fight wouldn't be much fun
I'm thinking to add in the new update a difficulty level, controlling all those aspects and also how many skillpoint you can assign to the manager, I think is the best solution.