Exactly, therefore the desaturate filter should not do anything. However, from your screenshot I see that the image gets corrupted, which is clearly a bug. I will fix that asap.
Concerning the luminosity mask, if you want to select a band in the luminosity you can do the following: add two points in the intermediate region, pull them to the top, and them move the two extreme points on the bottom axis and very close to the intermediate ones:
This will give you a sharp rectangular shape without any distortion.
The reason why you get distortions in your case is that the spline interpolation function tries to compensate the sharp transition. This does not happen if you directly move the extreme points…