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I use it on D800. I test it at approximately 3m.

It is normal on a Nikon camera with Macro lenses. The camera will report what it thinks the effective aperture will be. Some find this confusing, and for instance Canon does not approximate the effective aperture, but just shows the selected aperture setting. So with Canon you can only figure out that light gets lost at closer focus distances, by looking at the exposure times.

 

If you test it at infinity, you will/should see f2.8. If you test it at MFD, you will probably see something towards f5.6?

 

My old and lovely Nikkor AUTO 55mm f3.5 shows it on the lens barrel. It is a shorter lens, so not totally comparable as to when the bellows effect starts to have a significant impact (the longer lens has more magnification at the same distance), it becomes f4 at 0.7m, f5.6 at 27cm, f5.6 at about 23cm and f7.1 at 21.4cm (when it delivers 1:1).

 

You get even smaller effective apertures when using bellows, that is why it is known as bellows effect.