And is this the same phenomenon we see when colours appear less saturated during the middle of the day versus dawn and dusk (when luma contrast is reduced)?
Because you did not also increase the chroma contrast. As a 1st order approximation the perceived saturation goes with the chroma : luma ratio, best kept constant.
No, that would be due the Hunt effect, and it is a matter of human color appearance versus simplified math. https://en.wikipedia.org/wiki/Color_appearance_model