and that is what it should do, it responds to the vehicle's dynamics, not your input. A brake controller's responsibility is to stop the trailer at the same rate as the TV is stopping.
Proportional braking is about applying a smooth braking effort on the trailer in tow. The trailer's brakes should apply a stopping effort to slow the trailer just as the TV applies the braking effort needed to stop itself. How much braking effort is the proportional part of braking. A factory brake controller will always be superior over aftermarket simply because it has additional information about the vehicle's dynamics through the CAN bus.
Locking the trailer's brakes is not an indication of the ability of a brake controller. Locking the trailer's tires doesn't stop a trailer, it could and will likely cause the trailer's loss of control. Take a scenario, pulling a loaded car hauler on wet roads and you have to panic brake. The TV's ABS system kicks on to maintain control of the TV and this information is available to the factory brake controller which responds by adjusting the PWM brake signal to the trailer's brakes. An aftermarket controller doesn't have this information available and can't mitigate this scenario thus applying full braking power to the trailer. Depending on the gain, your trailer will likely be passing you.
I've used a Prodigy II for years, it is an excellent controller but the ITBM's in my RAM's outperform it, not because it isn't a good controller, because it doesn't have all the vehicle information available.