I am going to assume that the viewfinder diopter adjustment was NOT set to your vision and that the images did not appear to be in focus. I have an X-T1 with the XF 35mm f2.0 and the XF 18-55mm f2.8-4.0 and the both lenses focus instantaneously and spot on in that circumstance - even if I step into a darker room. I find it amazing just how sharp the exact point of focus is when I shoot either lens wide open and inspect the images in Lightroom...every time!
Just a thought...