Brake Assist will apply brakes in full panic stop mode if you move your foot quickly from accelerator to brake pedal. If Brake Assist engages at higher speeds (50 mi/hr is sufficient), the brakes will stay fully engaged until the car comes to a complete stop. That can be disconcerting. The only way I have found to get the brakes to release before coming to a complete stop is to depress the accelerator pedal. Nissan considers this "normal" and will do nothing to change the brake assist action.
It sounds like either the controller or sensors have failed and it is applying brake assist. Do the brakes release when the car is sitting still or do the brakes remain locked? Since the intelligent brake controller is integrated with the inverter and traction motor controls, I would argue with Nissan and your dealer that the entire system should be covered by the EV power train warranty (5 years or 60,000 miles in USA).
Gerry