I think you just found the worst case scenario for reflections, It doesn’t seem to be an issue with your model, I tried using a sphere and a cube with a sphere cut out, and while I didn’t see much banding, I did see some. Obviously this isn’t very noticeable once you uncheck Lower Hemisphere is Black, having black in the reflections makes the banding worse, and isn’t very realistic.
The banding occurs along the polygons, and having a lower poly model or normal maps would help reduce the banding significantly