I think there’s the issue. Following the color calibration module code, if adaptation is none, then in the R, G, B tabs R = X, G = Y and B = Z. Now the Y in XYZ is the luminance. If we replace the luminance by Z (~blue) in the mixer, the R letter goes to black because the Z component (and therefore Y in the mixing result) for it is zero.
Switching the adaptation to linear Bradford, the result looks much more sensible:
I wonder if the channel-swapping presets should indeed use something else than XYZ for the space.
Edit: the tooltip of the adaptation combobox says that “none” should use the pipeline working RGB instead of XYZ. Therefore I think it may be a bug here: darktable/src/iop/channelmixerrgb.c at c9e593666015b59b928b676f722446f5bc60acfa · darktable-org/darktable · GitHub
What do you think @anon41087856?