At least the surface that is generated when a 3d interapolation was confirmed has flipped normals. You can verify by copying/remeshing the surface and render one surface as point cloud, the other one as wireframe model. Switch on backface culling for the point cloud variant and you also may want to increase the point size. The "front" is culled, which is now easy to recognize with the wireframe surface as reference. I hope that the flipped normals also cause the ugly rendering, where it seems that a lot of triangles are clipped.