I'm just going on various comparative reviews I have read over a fair period of time where the AF between the A9s (and A7s) vs various Nikons (and others) is not as straightforward as might be indicated by how well they perform in the areas that tend to be emphasised eg birds in flight and eye detection.
Most recent (and I know its not a D850) is Steve Perry's D6 review where as well pointing out how much better the A9 was for flying Swallows, he also points out how much better the D6 was for focussing on a bear in shadows (both cameras with 600mm f4 lenses). Or the Z6 vs the A9 (not BIF) where the focus is faster on the A9, but the focus on the Z6 is more accurate.
Overall, there seems to be a fairly clear pattern that the comparative AF performance between cameras (and I have only put any effort into Sony, Olympus, Fuji) and Nikon, principally the D850, D500 and Z6) is not straightforward and which is "best" for you will depend on the range of subjects you photograph and the range of conditions you photograph them in.