It's a very good question!
I photographed braun hares lighted with an evening sun on the green grass where my Z9 didn't want to focus at all. I had a self-made very flat profile (like all in munis)
The hares were hell brown, a bit yellowisch-orange and the grass was green. The camera didn't see the animals and tried to focus on the edge of the field where light changed to shadow. It was a very strong edging.
So, for me it looked like the AF-System is a colour-blind and see the differencies in luminance only. The luminance of hares was approximately the same as for the grass, I think.
I checked on the colour wheel and other hue-luminance tables. It could definitely be the same.
For me, of course, the colour contrast was obvious. But for camera not at all. For camera the contrast was where the light changed to shadow.
Somehow I've got it and changed to neutral and AF was working as it must. It got more contrast in luminance.
It could be true that AF-system works only with luminance contrast? who can confirm or disprove it?
I think so also becasue the luminance-contrast will work in the darkness better. Remember about cat-eyes which can differ thousands shades of gray in the darkness and see much better than humans. And theoretically colour in the camera is a filter in front of the sensor. If I would be an engineer I would probably build AF on the luminance-contrast becasue it gets more data but I am not an engineer
That all would conirm that by flat colour-scenaries or scenaries with similar-colours (which are similar in luminocity) the vivid profill will work better becasue it accentuates the contrast.