exposure - What causes this dark halo around the sun?

I took a series of photographs just before sunset, directly into what was a very weak, watery sun - barely made you squint to look at either with the naked eye or through the viewfinder.

To the naked eye, the sun had a light halo surrounding it, diffusion by the atmosphere - which later turned out to be sand & dust being brought over by Hurricane Ophelia.

However, on the exposure, that halo became darker than the surrounding sky rather than lighter.

I'm aware the sun itself is completely blown out, & though overall I think it gives the picture an interesting look I'd just be interested in knowing how it happened.

No post-processing at all, other than defaults applied by NikonView & conversion to jpg for upload here.
D5500 18-300mm f3.5-6.3G - 300mm ISO 100 1/1600 f6.3

I tested other apertures/speeds, but all show the same effect to greater or lesser degree, not related to the aperture itself.

Answer

This can happen when there are strong light light sources in the scene (such as shooting sunrises/sunsets like yours), when Nikon's Active D-Lighting is on. Active D-Lighting (ADL) tries to help balance an image that has areas of high local dynamic range; essentially, it is a form of high dynamic range processing.

ADL slightly reduces the exposure, so darker regions near high dynamic range transitions can be boosted. There is also some tone curve adjustments that occur. Importantly, ADL is applied before/during the exposure. It is part of the image.

Caveat: CaptureNX2 can remove ADL if it was set to "Low". But any other setting, or any other processing tool (such as ViewNX2) cannot completely remove the effects of ADL.

Note that D-Lighting is not the same as ADL. D-Lighting is applied in the Retouch menu in camera, and if shooting RAW, can be removed/disabled by CaptureNX2 and ViewNX2 even if was applied in the camera's Retouch menu.