Hi Mustafa this is a great feature in the color calibration module. I use it a lot when color matching between images is important. Have you read the user guide? Here is the link if you haven’t darktable 4.6 user manual - color calibration
To use the area color mapping you open the image with desired color and set area mode to measure. Then when I open the next image I want to correct the area mode has been set to correct. This feature allows me to color balance images so well.
Now if I would like the color mapping to neutralize colors or match a grey card in an image I just set the chroma slider to 0.0. I feel set the chroma slider to 0.0 will give you the behaviour that you expect. There is no bug in what you describe. It is the expected and desirable behaviour of the module.