I have had this lens for several years, using daily for sports action. I post the pictures on my grandsons' Shutterfly share sites. I normally set the shutter speed to about 500 or 800 and the aperture to f/8 to try to get some depth of field. Then, I let the ISO float to get the right setting. I have never had the dark shots you are talking about. Am using an X-T2. Have you looked to see if there are any firmware upgrades to either your camera or to the lens? https://fujifilm-x.com/global/support/download/firmware/cameras/ and also the page with the lens updates.