Help with DFRobot ACS758 50A Current Sensor
From the looks of it, you are probably getting an offset reading. If you subtract this value (reading when disconnected) and then start / stop current, does it behave according to expectation? Also do you have the opportunity to calibrate using another amp-meter? At least compare at 10, or 20 amps, whatever current your amp-meter can handle. I am also curious how are you generating 50 amps in your test setup. 

On another note I can suggest an easier smoothing algorithm:

double SmoothReading, NewReading, Factor;

setup(){

Factor=30;

}

loop(){

SmoothReading = (SmoothReading*(Factor-1)+NewReading ) / Factor;

Display(SmoothReading);

}

The bigger the 'Factor' the smoother the reading, but also the slower reacting instrument.

This simple digital low pass filter served me well over the years. If you want to get real fancy you can make 2 or higher order filter. With higher order filters you get better noise rejection with faster reaction. To make 2nd order filter, you treat SmoothReading from the first filter as a NewReading for the second one and so on. For 3rd order filter I would divide Factor by 3 to get similar responsiveness.