I also have to experiment with focus priority when doing tracking with the topmost point in portrait mode, no way around it really when tracking a horse running towards you... my problem might be a combination of dirty af array, non-cross point and bad 6d tracking behavior. Good to hear from some people without super powers, now I've much more to go on than before :-)
I wasn't aware the horse was running toward you.... I don't have much success with those types of shots either! The 6D AF can handle side-to-side movement pretty well, but it definitely struggles with targets moving at anything over slow speed toward the camera
. I'd say my hit rate with those types of shots is pretty low, maybe under 60% depending on the DOF.
I could nail focus with any of the AF points on the kiddie rides at f/2.8 with my 70-200 using One Shot with no problem, but shots of my older son on some of the faster rides moving toward me was iffy with either AI Servo or One Shot. I really didn't even try these shots much since I knew they wouldn't come out.