Just to add another possible cause pointed out by someone at dpreview as posted by Bruno here, and that is with regards to the focusing sensor sitting at the bottom of the mirror box.
It seems to be sitting slightly shallower then what is on my D800, I can hardly see the 3 windows flames of the focusing sensor on my D800 when I was trying to blow clean them, and yet it is so visible clear on the D750. Could this be the reason that the issue was found on wide angle lens and not on telephoto because for WA lenses it has a more convex inner element and sit closer compare to that of the tele, and therefore is easier to cause reflection. As for shooting without a lens, we can clearly see it is definitely possible with light directed into the sensor surface.
