I have a Pimax 5K+ which I can't run in full field of view, because Elite makes an incorrect assumption that both displays are pointed straight forward. That's not the case for Pimax, StarVR, Xtal, who knows who else. At least the optics of the Vive Index are also canted. Point is, GetProjectionMatrix and GetEyeToHeadTransform may contain rotation as well as translation. Supporting it is generally easier than not; it fails because you're specifically rebuilding the same transformation from incomplete data.