OK, you can see the solution in the exposure module:
There’s a checkbox, saying ‘compensate camera exposure (+1EV)’. And then an exposure of +0.5EV.
- The first of those means that when you took the shot, you applied an exposure compensation of +1EV in camera (that is, you tried to make it 1 EV brighter than the camera would have used). This means that the module applies -1EV to compensate, making the image darker.
- The second setting means that after compensating for the exposure correction / bias used when taking the shot, +0.5 EV is added.
The end result is that, in your case, the exposure module is making the shot 0.5 EV darker.
So don’t panic, there’s nothing wrong. Just adjust the exposure manually as you see fit.
The exposure module is trying to compensate for the exposure correction dialled in the camera because people often dial a negative value there to avoid clipped highlights (making the image darker in camera, saving the highlights, with image brightness to be corrected in post-processing). However, filmic requires that you start with a photo that’s exposed properly for midtones (filmic is centred around midtones). Don’t let it fool you that ‘exposure’ comes after ‘filmic’ in the history: the processing order is shown on the first of the tabs on the modules block, under the histogram: exposure comes first, then filmic (the ‘pipeline order’ lists the first-to-be-applied module at the bottom, with the last-to-be-processed at the top).
So: raise the exposure, not caring about the highlights; then go to filmic, and raise the white level there to bring them back.
Please read this article (long, but extremely useful and important): darktable's filmic FAQ