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Apparently a camera matching colour profile emulates a camera which has adjusted colours in a certain way internally BEFORE it reaches LR.
And a mono camera matching profile emulates a camera which has mono converted the image internally BEFORE it reaches LR.
So Camera Monochrome works as if you'd imported a mono JPG rather than a colour JPG, even when it's a Raw. This seems to me an odd, backwards step.
And the interface does not distinguish these different sorts of profiles clearly.
And I don't think LR should be substituting profiles, silently, when you haven't asked it to, anyway.
My own approach to B&W conversion continues to be: leave the image in colour mode!
This retains all of LR's adjustments available as normal, based on any camera matching OR non camera matching profile you like. (I am fine about not getting an Auto B&W mix, preferring to do that myself).
I find this best via a simple preset (one of my very few) which sets all HSL panel Saturation sliders to zero. Your HSL panel Luminance sliders now become in effect, the "colour mixer" that you would have otherwise seen if in B&W mode. And Hue sliders, WB and Vibrance and such still subtly modulate the input which this "colour mixer" is seeing.
Many thanks, Richard - using the HSL panel is a good hint!
The "Camera monochrome" profile seems to be odd (or broken) in my case - my workaround so far was to use other monochrome profiles.