If the background is what is troubling you, you may add -mask mask_image of your whole brain while calculating fa from the tensor. tensor2metric — MRtrix 3.0 documentation
That does look odd for an FA map… May I ask whether these were acquired on a Phillips system? If so, this could be related to this issue – which I note we are not always able to detect correctly. If you can post the full output of mrinfo <input image> -dwgrad, we might be able to spot the issue if that’s what it is…