What is wrong with this mesh?

[Amir_Habibi (Sag)]

Some mesh faces are not being displayed , I’ve even tried :
M.Vertices.CullUnused();
M.UnifyNormals();
M.Compact();

MeshFace.3dm (61.5 KB)

[laurent_delrieu]

ExtractNonManifoldMeshEdges or Exploding the mesh suppress the problem.
As you have edges with more than 2 faces, it is not possible to decide what normal to take.