A colour wheel from Wikipedia with sigmoid in rgb ratio mode:
In per channel mode with preserve hue = 100%:
Accroding to darktable 4.4 user manual - sigmoid, the two should be equivalent:
preserve hue (per channel mode only)
Choose how much to preserve hue – 100% preserves the spectral hue of the image (identical to using the “rgb ratio” color processing mode)
Is the documentation wrong, or is that the intended behaviour, and a bug has crept in? @jandren, perhaps?