Yes, the sphere works just fine. I ran across this problem because I needed to use a water surface plane for a scene I'm working on. It includes a stream bed that falls half a meter across the width of the scene, about 150m, enough to be noticeable if the water surface doesn't follow. Using a slightly inclined water plane seemed to be the quickest solution. It is also a little over 1km from the scene origin. So in this very specific instance the mask distortion was noticeable. I just wanted to determine the cause. Now I know!