I am not an expert at the problem you have here so I defer to others who may answer. Here are my settings which work well for me . Looking at yours I feel you are achieving nothing advantageous setting image quality to 100% but that is not your problem. Maybe the chroma subsampling settings or image intent settings are the issue? But I really don’t know.
What is your display profile set to in DT… vs what profile is being used by the OS and perhaps by the viewers… are all set the same?? Its best to specify them everywhere if possible to be sure you are comparing apples to apples…
Apart from colour management issues (viewer colour managed or not): there are some modules, whose effect depends on resolution; this usually manifests as brightness/contrast change.
On the master branch, which I use, there is a new button to force this in the darkroom, without zooming in to 100% (at the expense of performance), but I don’t think it exists in 4.6.0:
Do you get the problem with other images (say, landscapes with colour)? The problem seems to be more frequent with contrasty B&W.
To see if that affects your image:
zoom in to 100% - does the image change (unfortunately you won’t see the whole image)?
when exporting, you have the option high quality resampling. When enabled, it processes the image ‘at 100% zoom’, and scales down the result; when disabled, it scales down early, which means you get better performance, but the same problem that the non-100% view of the darkroom has may occur. If that is the root cause of your problem, exporting at a size similar to the dimensions of the darkroom editor area, with high quality resampling disabled, should result in an image which looks like what you see in the darkroom. I see you have already tried that option, but were the dimensions similar to your darkroom editor’s preview? Since your first screenshot, which is not full screen, is 1961 pixels wide, and the viewer showing the exported image shows an image width of 3340 pixels, that could be the case; but then shots of your export dialogue show a target resolution of 1200x1200 pixels with high quality resampling enabled, and then 1080 x 1080 with it disabled.
If the experiment shows this scaling issue affects your image, you must use the 100% zoom to check brightness, and export with high quality resampling enabled. Alternatively, you must judge brightness without zooming in, and export at resolutions close to the darkroom editor preview’s dimensions, with high quality resampling disabled.
Lastly, do you, by chance, have options like prefer performance over quality turned on?
Sorting out the problem would be more beneficial in the long run, I think.
You can of course delete darktable and revert to a saved version of config + database. Darktable creates a backup of the database when you upgrade. See INSTALL last version - #9 by kofa and INSTALL last version - #11 by kofa (on Linux it’s the same, but the files are in ~/.config/darktable).
If you edited the XMP files, darktable 4.6 may have updated them in a way that 4.4 won’t be able to read. But 4.6 is very nice, I really suggest that you let us help you figure out and solve the problem.