He, he, he. It's a Preview rendering problem.

You are using a two-sided mesh, so you have a front face and a back face. The vertices are not weldedand so each face has its own set of vertices.

During the rendering process the render engine must decide which face have to render. The faces that have the normal looking forward are rendered, but for each point there are always two faces with forward normals in the overlaping region and the renderer is picking the wrong face. The renderer seams to be biased and not symetric, in one side picks the right face and in the other side the wrong face. It also looks as the distance of the faces are not used for the decision of picking the correct face (No Z-buffer ???)

Inverting the normals of the mesh solve nothing, the distance between the front and back face is very small.

In the final render there is no problem, the renderer always render the correct face

Stupidity also evolves!