I’m on mac 10.12.6, using darktable 3.0.2
Using a second instance of channel mixer, I set dropdown to ‘grey’ and enter in rgb values 0, 0.33, 0.67, and set blend mode to lightness. I then click the little menu to store preset, but as soon as I do, the image turns black and white, and changes my rgb values to 0.28, 0.41, 0.31. These are the values for the B/W Ilford FP4 preset, so for some reason channel mixer is selecting this preset on mouse hover, without actually clicking on it.
The issue was solved by turning the channel mixer power button off, then back on again. But of course, one shouldn’t have to do that.