On a heavily used Nikon D750 w/ AF-S NIKKOR 24-70mm f/2.8G ED:
When using live vision, ussualy while holding the camera up over my head, autofocus focuses nowhere. This happens right after having shooted a correctly focused photograph using viewfinder, with slight subject distance variation.
When doing so, I hardly ever want a shallow DoF, as I'm focusing on groups of people, so F-number is always between F5.6 and F16. Focusing mode is always AF-C and center point, and I don't use shutter button focusing, I use only dedicated AF-ON button focusing. So it should just focus at center of image as I press AF-ON button, and keep this focus distance until I shoot.
In most notorious cases, it seems to be getting stuck at its shorter focusing distance:
At some other situations, it fails to focus the central situation, setting focus on closer, peripheral subjects (as if I was using multiple point AF selection, what I am NOT doing):
Is this caused by a fail, A D750 common issue, or a mispractice?