I just have observed a strange behavior in Darktable (Fedora 32, darktable-3.2.1-1.fc32.x86_64): when looking at the image with 'fit screen', it is much darker than the 100% view. After saving the image in 16bit TIFF and opening it with GIMP, I can see that the 100% view is much closer to what GIMP shows.
More: in Darktable, going from 100% to fit screen gradually, I can see the image getting darker and darker.
Why is this? Mabye a config thing? A Bug?
Here you can see screenshots with the two views of Darktable, and the 16 bit Tiff opened with GIMP (or with Darktable itself!):
EDIT: Just filled a Bug report.
EDIT2: The problem is MUCH more serious than I thought, and it is not related to zooming the view: Darktable saves an image that is underexposed with respect to the one shown at the screen. This does not happen with TIFF files, but do happen with my RAW files.
EDIT3: As you can check it by yourself, Darktable team was extremely rude and closed the bug report without a reason. So do not expect this to be fixed anytime soon. I'm glad that I use RawTherapee!