This is not so much a question as a comment, though if anyone has insight on the subject, feel free to weigh in.
I've progressively worked with a number of cameras over the last few years while doing sports photography: 7D, 7D mk II, 5D mk III, 1Dx, 1Dx mk II. I have noticed behavior which seems to contradict the manual and the Canon tech notes, and it doesn't seem to be my misreading of the descriptions.
Sometimes it's useful to set one of the various zone focus modes in AI Servo (I tend to use the smallest zone), when I'm shooting a small group of athletes. I know there are limitations, like keeping all the focus points within the area of the potential subjects, but occasionally it does what I want - focus on the group rather than my having to manually, relentlessly keep the center AF point on a subject which is in the middle of said group. However, Canon has published in multiple places that their zone AF tends to focus on the nearest subject in the focus point group, and after about 8-10 years of shooting with the various high-end cameras (literally tens of thousands of similar shots), I have come to the conclusion that this is not so.
If the contrast or pattern of a more distant object which happens to be within the zone is even a little higher than that of the closer object(s) - such as a patterned athletic uniform, a brightly colored banner or a set of bleachers - it will grab that object in preference to any closer one. It will do this even if it's barely visible between much closer subjects. It will often, perversely, focus on the rearmost in a group of athletes even when the zone boundaries are entirely within the desired group area (yes, I know that the AF points are larger than the ones shown in the viewfinder). If I had to quantify the behavior, I would say rather that it seems to focus randomly between the front and rearmost subjects, if there is no highly contrasty subject - and it always locks on the subject with highest contrast if it locks at all. (There's a separate issue with the cameras - all of them - indicating focus when there is nothing in focus, but that's not the subject of this observation) It doesn't seem to matter which of the several lenses I use either; from the 24-105 f/4 to the 70-200 f/2.8 II.
AF customization settings don't seem to matter (I've tried changing each of the 3 parameters separately as well as together); the best you can hope for is that there is nothing inside your zone which is behind your intended subject. Otherwise you're better off using single point AF and being careful (because if the AF point slips off the subject it too will jump to the background). Single + expansion points does the same thing even with tracking sensitivity set to minimum, because I've tried it - it seems to behave almost exactly the same as zone, though with fewer points to manage. It just loves higher contrast objects under the center AF point. It's not an AF microadjust issue either; single point, when I get it accurately on a subject, is dead on. What bugs me is that everyone just seems to repeat what Canon says about zones, but no one seems to actually have tested it and confirmed that it behaves the way they say. Every single one of the cameras I have owned which supports zone focusing has behaved the same, and no tweaking of the tracking parameters seems to help.