Even Nick's exported dark version shows a lot of posterisation.
(ALL of the light versions, from anyone, are missing the point : the problem mostly goes away as the image gets lighter)
Dark areas have some kind of artefacts or posterization. I don't know why.
I think perhaps I've lost the plot! I was thinking of this area looking as if it is posterised in the originally posted version. I'm thinking of "posterised" as in multiple hue/brightnesses going to a single value, or very few values. ("
Posterization of an image entails conversion of a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another." Wikipedia.)
My hypothesis was that the originally posted version, which was very small at 35KB, was presumably highly compressed, with the compression causing posteristation in areas with only slight/subtle variations in brightness and hue.

NOT MY IMAGE - jamiebonline as posted posterisation outlined by
gardenersassistant, on Flickr
But this is not a dark area (well, not relative to the rest of the lady's face, hair and dress). Can you indicate exactly where you are seeing this posterisation/artefacts, and/or give a magnified view of some of it?
As to the area I have outlined, to me it does look posterised in the originally posted version. However, on looking closely at the original, I don't think it is. (For the avoidance of doubt, I am working on a hardware calibrated screen that shows all levels in the
Lagom black level and white saturation diagrams and has gamma very close to 2.2 on all three of the gamma diagrams.)
Here is a pair of screenshots, on the left, at 100%, is the NEF file as imported into Lightroom with the only adjustment being default colour noise reduction. On the right is the same 100% view, this time of the JPEG version which was exported from Lightroom, with no other adjustments, and reimported to Lightroom. As suggested by Ulfric, these have not been lightened at all. (You might need to click through to Flickr to see the full size version of this composite image, which is 2652 pixels wide.)

NOT MY IMAGE - jamiebonline NEF Default Colour NR 100pc screenshot composite by
gardenersassistant, on Flickr
Here is the same pair, but at 200%.

NOT MY IMAGE - jamiebonline NEF Default Colour NR 200pc screenshot composite by
gardenersassistant, on Flickr
I think what we are seeing in this area is the impact of the lady's makeup, which was somewhat confirmed when I asked my wife (not a photographer) to tell me what she saw (without telling her why). She explained all the colour and texture variations, at length and in considerable detail, in terms of what makeup she thought had been applied, and how.
Now you tell me I'm looking at the wrong things, and I'll go back to lurking. (Maybe

)