The regenerative braking forces are very strong in these cars. So much so, that if left unchecked they would interfere with proper ABS implementation.
If the vehicle senses an emergency situation, it will turn off/reduce the braking regen in order to allow the ABS to function properly. This creates the sensation of acceleration. Often if your foot is not in emergency mode (ie. max pressure on the brake pedal), it is necessary to press harder on the brakes.
I used to feel this quite often when the car was new (HiHy). Now I barely ever feel it. BTW, the car stops on a dime when I need it to.