Darktable 3.0 Exif on export

I did some testing with an older image. Aperture f/2.8, ISO 100, focal length 24mm, exposure 1/1600. I set some values with variables and checked with exiftool after exporting. The result:

$(EXIF_APERTURE) → exiftool is reporting a value of 1.0, should be 2.8
$(EXIF_ISO) → exiftool is reporting a value of 100 = correct
$(EXIF_EXPOSURE) → exiftool is reporting a value of 1, should be 1/1600
$(EXIF_FOCAL_LENGTH) → exiftool is reporting a value of 24 mm = correct

Is this a bug or is my understanding of the export options wrong?

Regards,
Marcus