While making a custom model I noticed some unusual artifacts when making two thin faces in the same plane, To further test this I created the attached Resource Pack which renders each pixel in the default 16x16 textures as mini-cubes, overkill; but this re-creates the issue I saw earlier perfectly and it is much more pronounced.

Additionally it also shows that the issue only affects the top & bottom faces.

I also tested with and without "Smooth Lighting" the issue is only visible with "Smooth Lighting"

I included a screenshot of the issue demonstrated via the resource pack, besides the custom resource pack to demonstrate the issue all game settings were default, and was a fresh installation.