As the global color picker runs at the end of the preview pixelpipe, it receives data in display color space then converts it to histogram color space. If you are using a display color space which is not “well behaved” (this is common for a device profile), then colors that are outside of the gamut of the display profile will clip or distort.
(emphasis mine)
Maybe if your working space is linear Rec2020 (which is darktable’s default), even if the colour is inside the sRGB gamut (e.g. your input is sRGB, and you don’t enhance/boost colours), the display profile distorts something. What happens if you set your display profile to sRGB, too?