Pixelated smooth lighting on 3D models

The top and bottom faces of extra polygons added to the block model have pixelated lighting/shading when placed directly next to the North or South of a wall. This doesn't happen when placing the block directly next to the East or West of a wall. This only occurs when smooth lighting is on.
Note: This block model has many polygons.

What I expected to happen was to perceive normal lighting like every other full block in Minecraft.

What actually happened was that the top and bottom faces showed a pixelated lighting/shading.

(For your ease I have narrowed down the models in the resource pack to those that are relevant.)

Steps to Reproduce:
1. Use the Resource pack containing the cobblestone.json block models attached to this bug report.
2. Place a cobblestone block or slab to the North or South of a 3-block heigh wall. (Or reproduce attached images)