My display profile is currently set to “System display profile”. If I change it to sRGB, I get a very small difference in the value of red = 254, 10, 10 (compared to 254, 9, 10 when set to “system display profile”). so basically no real change.
But I’ve just discovered something: the default gamut compression setting in the color calibration module is 1.0. When I reduce that to zero, I get the values I would expect (255, 0, 0 for red).
I had set color calibration to bypass so I didn’t think it was having any effect on the image, but the default gamut compression does have an effect. I will do a few more tests, but maybe this was the issue all along! Sorry if I’ve wasted everyone’s time…