When you change the value of one color channel in HSV/HSL/OKHSL/HSLuv/CMYK, the values of the others also change. Only RGB works fine.
Is this a bug or is it supposed to be like this? I didn't notice this in other programs. How to fix it?

I think its common to see the saturation values change with variation in lightness/luminosity. The Hue channel is likely right on the borderline.