Hello @hyperviktor,
you are facing this behavior because of the color_picking role which is set by default as Output - sRGB.
You could switch to ACES - ACEScg or Utility - Linear - sRGB if needed.
With Output - sRGB as mixing color space, 0.5 becomes 0.303. That’s completely normal. On a recent update on my website, I have tried to list all pros/cons for different mixing color spaces in Maya.
Otherwise, you have this thread on the same topic.
Sorry for the late reply,
Chris