This is almost certainly a user error (having in mind the skill of the user), so some quick advice would be welcome.
Some of the raw images from my Canon G7 X Mk II appear to be heavily distorted (fish-eye-like), including the appearance as if an ill-chosen lens hood was on the camera, when imported into darktable in Linux (Mint 20, Cinnamon). This does not happen in Windows. darktable 3.2.1 in both environments.
If I discard the history in lighttable, and/or delete the associated .xmp, the image in the file manager view immediately looks normal, until I open it in the darkroom. Even if I go back in the history stack to the ‘original’ state it still has the distorted appearance.
If I remember it right Canon G7 X II needs lens correction. Same as RF 24-240 or G7 X at 8 mm. Check the lens correction settings.
The reason why it looks normal in Lighttable is because you are seeing the embedded and lens corrected jpeg.
This sounds plausible - until I notice that if I return to lighttable from darkroom, with the distortion uncorrected, then the distortion also shows in the file manager view in lighttable - until I discard the history stack, as previously described.
The good news though is that applying the lens correction module immediately corrects the problem.
The not so good news is that I was not sufficiently rigorous in my testing in the Windows environment: on closer investigation there is no difference in behaviour between the two operating environments. Lens correction is necessary and effective in both.
About Lighttable, check “don’t use embedded preview JPEG but half-size raw” in the settings. It will be slower but Lighttable will not show you the embedded JPEG files anymore.
About Darkroom, create a lens correction preset to always apply lens correction for raw files from G7 X II