So, I figure that I'll split the rendering of tall cells into two draw calls, clipped by a custom plane.

Found a strange bug. It only lights some of the faces, depending on view. Up close, everything is as expected. It's when far away that the lighting bug shows up.

Any ideas?

There was a bug in the Phong shading in my code. The bug has been there for weeks LOL. I'm surprised that I didn't notice it until now.