A correctly-functioning D lens set at the smallest aperture should operate just like your G lenses with the aperture in sub-command-dial mode. A few things to look at after you put f6 back at its usual setting:

First, make sure the aperture ring is set at 22 and the aperture ring lock is engaged. (It's a little switch just below the 28mm 1:2.8D marking on the barrel.)

Second, check that the camera is getting a good read on what lens is installed. Install the lens, go to the Setup menu, select AF Fine Tune and then Saved Value. The screen will show what lens the camera thinks is attached. If it's wrong, the lens contacts may need cleaning or there could be a problem with the lens itself.

Third, check that the mechanical linkage between the lens's aperture ring and the camera are working properly. At about the 2:00 position at the edge of the lens mount (looking from the front) is a lever that can be moved by hand. This is what tells the camera the position of the aperture ring. Remove the lens from the body, turn the body on and half-press the shutter button. The display should show Δf/0 and the number should increase as you move the lever.