Using Darktable 3.2.1 I get this jpg with default settings.
Then, I get this jpg with VNG4 demosaicing algorithm, default "sharpen" option and automatic exposure correction.
I don't believe Darktable is to blame here. Maybe you are pushing too much some settings, since histogram shows an underexposed photo:
Playing with 'L' in Lab color ...
That is the autofocus drive mechanism and it is supposed to be below the surface anytime the camera is not trying to focus a lens.
The lock/unlock pin is also raised as it is supposed to be; but you mentioned that the release button sticks in the down position. It is possible to bend the release pin so that it binds in the opening (I've done it).
Are you ...
This is the RAW image cropped but otherwise untouched, saved from Nikon's own ViewNX-i as a 100% quality jpg.
It shows some hint of interference, as we're right down at pixel-peeping, but not the extra 'crunch' darktable seems to have added. The colour fringing is vastly reduced too.
It has, of course, had the snot cropped out of it - it's down to 493x362 ...
Things to consider…
35mm in a DX body will let you take full body shots from reasonably close.
This will, however, give you 'short legs syndrome' if you're not careful. Shooting from any lower than chest height to try balance the leg length starts to get a bit 'up the nose'.
Your bokeh will be harder to achieve on a relatively wide lens, even with the ...
Every stop you increase the ISO you loose a stop of DR from the highlights.
But luckily for you the D3400 is nearly entirely ISO invariant and there is less than half a stop benefit to raising the ISO as compared to just recovering the underexposure in post. And a one half stop difference is barely ...
Based on the comments on the question and some answers, I figured out the issue. The issue can be removed by enabling Match Greens: local average. If you also use AMaZE demosaic algorithm, the artifacts are completely gone:
This is the result of Lightroom, default settings +1 Exposure:
Looks much more reasonable to me, with a little but of colour fringing which could be removed easily.
Conclusion: the demosaicing algorithm of Darktable has trouble with the fine details in the feathers.
All you can really do is exactly the same as you would for a photo: protect the highlights, and lift the shadows in post-processing. In your case, that probably means dropping the ISO.
However, I suspect you may well find the results of doing this with your D3400 aren't of the quality you want, because the sensor has only a limited dynamic range and once you ...
Try this in good light (contrast detection in live view needs more light than the sensors used when looking through the finder) and with a target within the range of the lens, i.e. not too close to the camera.
I have seen the same issue with a D7100 (& Nikkor 18-140mm ) this week. Just as described the D7100 operated as normal and it continues to increment the image numbers. Lucky for me I spotted this early. I removed the battery and removed both Sandisk SDHC cards. After replacing the battery and then the 2 SDHC cards normal operation resumed. I am on the ...
mine was solved by removing the neewer battery grip, then I had a think, and popped a spare battery into the grip and problem solved... so with mine it seems that the camera does not like a grip without a battery. just have to solve the RAW files not read by PS6 now !