This is most likely a demosaicing/rendering artifact made more visible by fine feather detail and some underexposure, not a sensor fault.

From the shared tests, the RAW itself shows only a slight hint of interference when rendered in Nikon’s own software, while darktable can add extra “crunch” depending on settings. In darktable, the fix reported was to enable Match Greens: local average; using the AMaZE demosaic algorithm removed the artifacts completely in this case.

A few practical points:

• Fine repeating detail like feathers can trigger false detail/maze patterns during demosaicing.
• Underexposure can make artifacts and noise more obvious when you brighten the image.
• Some sharpening or contrast adjustments may exaggerate the effect.

So the best first step is to change darktable’s demosaic settings, especially Match Greens: local average, and try AMaZE. If needed, reduce sharpening and avoid pushing the file too hard. Comparing with Nikon’s own RAW software is also a good sanity check.