Thanks for that @danny. I’m still very new to darktable and not familiar with the softproofing icon (which I have now just read up on). What I thought you meant was the ‘set display profile’ feature, which is the little monitor icon bottom-right of the image thumbnails in lightroom. When I click this and hover over the ‘display profile’ setting there is a clue as to where you can add your own profiles. In my case I’ve put the profile I created with my calibration software into
C:\Users\Walter\AppData\Local\darktable\color\out
So now that profile appears in the dropdown list and I can set it for the ‘display profile’ and ‘preview display profile’. The settings are permanent and survive restarting darktable.
I think given the uncertainty about which profile dt is loading, this solution is probably easiest to live with until such time as the bug is resolved.
edit I now realise that what I have done is exactly what you were suggesting, but coming at it from lighttable as opposed to the darkroom softproofing icon. Every day is an education!