I have this exact same problem. The strange thing is that both X and Y act the same way. I’ve measured the resistance across the range of motion and it does indeed go to a limit value around 50% of the range of motion. I’ve found that the middle is around 4k ohm, max is 5k and min is 0. My guess (but I’m new to all this) is that the wiper should be reoriented so that the middle would be at 2.5k, but I’m not sure if that’s possible without breaking it.

X_pinValue = analogRead(X_pin);
XVal = map (X_pinValue, 0, 1023, 0 ,255);

works

You are stating that your xValue is = analogread, but also stating that is it = map.

X_pinValue = analogRead(X_pin);
XVal = map (X_pinValue, 0, 1023, 0 ,255);

works

I was added this to my joystick arduino sketch
int xValue = analogRead(joyX);
xValue = map(xValue, 0,1023, 0, 255);

but still on the serial monitor I see values between 0 and 1023; instead of between 0 and 255. Do you know how to fix this problem ? thank you.