Did you check that your area color mapping has a neutral sample (chroma = 0?). That setting survives resetting the module, even restarting darktable. It often fools people (it did me, when I wrote the original version of this text).
With DT master (but also on 4.8), the new white balance + color calibration combo uses white balance in the recently introduced as shot to reference mode.
That gives this result:
I normally switch color calibration to in camera (I even have an auto-applied preset for that). It sometimes introduces a tiny change, if color calibration thinks you are close to one of the standard illuminants, and pulls you there. No difference this time:
Picking from the board is very close to neutral:
