Interesting. I didn’t know you could lose your power assist if you let your foot off the brake in a situation like that.
Yes, for a power assist that is run from engine vacuum (very common), this can happen. You don’t actually lose braking, it just becomes much harder to get it, to the point that the weak or elderly can’t manage it. Pumping the brakes, as opposed to simply standing on the pedal, is the issue here.
Park your car and shut off the engine. Step on the brake a few times and note the pressure it takes. After a few pushes, the pedal gets “harder”, that is, it takes more pressure to operate it as the vacuum in the accumulator is bled off.
At full throttle, engine vacuum drops down to almost nothing. I remember my Dad’s ‘49 Plymouth with vacuum operated wipers, which slowed to a crawl under acceleration, but operated at full speed at idle or cruising.
At full throttle, there is little chance for the vacuum in the accumulator to build up so that the power brakes function normally.