Material Edge Aliasing / Black Line

I have a material that is using vertex normal to render to each side of a cube. The top of the cube looks good. The bottom has a black line on the edge between the sides and bottom. This line is very thin because it almost disappears at certain angles, but noticeably flickers during gameplay, mostly when the camera is moving. I have the texture sampler source set to “Shared: Clamp”, which makes it better, but doesn’t resolve the issue.

Any ideas what is causing this and/or how to fix it?

Thank You,