Here's a related behavior that looks like a bug:
When picking normals or world position directly from the 3D viewport, the normal/position Z coordinate isn't being picked properly. For normals it's always zero and for position it seems like negative values are being clamped. And on a large segment of the test sphere the x/y/z normals were locked to 0/90/0. The tooltip shows Z depth instead of PosZ/NZ. Maybe that's (part of) the problem?
The sliders were created as user controls with some manual editing as described here.