Glad the D6 seems to work out very well for the both of you!Nice sharp images!
My guess is that the (any) face detect system is very challenged with the faces being a very small area of the frame, there are very limited data for the system to work with.
Right, as far as I can tell, it won't tell the user whether a face was recognized or if it just did the regular group-area which focuses on the closest subject within the group area. What it can likely do is recognize what appear to be human bodies (based on the matrix meter RGB images) and then estimate where the face would be and if there are any group area focus points in that area. I haven't used the regular group area much in the D5/D850 as it seemed to have a high propensity to focus on whatever is closest: hat rim, arm, or in the case of facial close-ups, the nose. So with these cameras I mostly used 9- and 25-point dynamic area. It doesn't prioritise closest subject so it is easier to avoid arms, noses and subjects cutting the line of sight, but it also has the downside of high probability of slipping to focus on something in the background if the photographer is not able to hold the primary point on the subject. For a subject that is small in the frame the old group area worked well but was fixed size. These modes required a lot of juggling the focus point around manually. The new custom shaped and sized group area with face priority on produces a high rate of keepers for me while not requiring a lot of work in moving the focus point around depending on which way the skater turns. I don't really know how much effect the face priority has (it doesn't always find a face even when present, that much is clear) but I am happy with the results and will try to do some tests to see what it is doing.
The focus with the 200/2 II is very fast but it used to run around a lot when using older cameras (it would always be going somewhere, following some small changes in focus sensor data) but it's very stable with the D6 when using custom group area of sufficient size, very accurate and no restless behavior. I think it's the nicest lens I've used in terms of how the images look. Of course, too much of a good thing, and Nikon seem to have discontinued it. I understand it is not a long lens and it is heavy but in terms of results it delivers a unique look. I read that Nikon justified that with the high iso capabilities it is no longer needed for photojournalism. This isn't about a "need" in terms of getting "a picture" - no, a 200/2 is not "needed" for that but it does deliver results that differentiate from the many 70-200/2.8 lenses that are everywhere. Why then make the 58/0.95 if they only want to make lenses that are "needed"?
105/2 DC, 135/2 DC, 180/2.8, 200/2: discontinued, what is in their place? 70-200/2,8, 70-200/2.8, 70-200/2.8 and 70-200/2.8. How is one supposed to make images that don't look like everyone else's, if only one option is provided?