Back in the day before the Black-and-White and Color Toning tools existed, one would make an image b&w by desaturating it in the L*a*b* tool (not the same thing as desaturating it in the Exposure tool, as there was special code that would make R=G=B when Lab chroma was 0, and I hope I’m not bullshitting you as this is from memory), and the a* and b* curves were used to tone the image. Then Black-and-White and Color Toning came along and we can now do things properly. I suppose the reason a* and b* tone your b&w image is that that part of the code was left alone when the new tools were added to provide backwards-compatibility. And I suppose that as such, it is not possible to make R=G=B when using the a* and b* curve despite enabling the B&W tool.
I think @heckflosse would be able to confirm (or smite).
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