Hmm, I posted a missive about the overall mechanics somewhere recently, but not in this thread. I’ll try to find it. In my travails, I haven’t found a description that doesn’t devolve into color theory. Not that it shouldn’t, but the mechanics tend to get lost in all that.
Now, I really don’t know RawTherapee that well, but I’ve seen some indication in this thread that ‘Luminosity’ was selected. Well, in any underlying mechanics, anything I know to be luminosity affects all three channels. Thing is, I don’t know what it means to select a single channel for the curve, but also have luminosity selected. The two together don’t make fundamental sense…
I wrote my own software, which includes a curve tool where an individual channel can be selected. I know for a fact it doesn’t touch the other channels when a particular one is selected. 
Now, I just opened my RawTherapee AppImage, went to the Color Management module, and found that I do have ProPhoto in my working profile list…