After reading the OP, I knew the problem right away....
The meter (even on spot metering) will average the exposure to be 18% gray, so any darker skinned person will be lighter than they really are IF.... they they take up most of the frame, or you spot meter their face.
You need to either use a mode that allows you to give less exposure (darker) with EC, or use FULL manual (including manual ISO) and underexpose by at least 1 stop. (use the light meter bar for that "-1")
If you have a live view on your D750, you can just dial in -1 and see the change.
With lighter skinned people, a +0.3 or +0.6 may be enough to bring-up their skin tone to lighter than 18% gray in the same photo setting.
Sometimes it's better NOT to allow the camera to decide any of the exposure settings! Instead, the photographers experience will decide what is needed to do!