Scripted mesh w/quad topology - material-per-submesh not working ?

I'm trying to create a mesh with submeshes in script using MeshTopology.Quads , before subsequently setting distinct materials per submesh via the materials array. Code is similar to that below, whereby I've simplified things just to be an example mesh with 2 submeshes and 2 materials.

Per the screenshot, it appears only one of the submeshes is correctly being rendered with a material - the other appears to have a 'neutral' appearance indicating that the material is not being applied to the second submesh, despite being correct.

Interestingly (and perhaps tellingly) if I set up a similar example uses the more 'standard' triangles topology, it works perfectly - so I'm wondering if material-per-submesh for quads is simply not supported by Unity?

         GameObject myObj = new GameObject("Object");
         MeshFilter meshFilter = (MeshFilter)myObj.AddComponent(typeof(MeshFilter));
 
         Mesh mesh = new Mesh();
 
         Vector3[] vertices = new Vector3[]
         {
              new Vector3( 1, 0,  1),
              new Vector3( 1, 0, -1),
              new Vector3(-1, 0, -1),
              new Vector3(-1, 0, 1),
              new Vector3( 1, 0,  3),
              new Vector3( 1, 0, 1),
              new Vector3(-1, 0, 1),
              new Vector3(-1, 0, 3),
         };
 
         int[] indexes1 = new int[] {0, 1, 2, 3};    
         int[] indexes2 = new int[] { 4, 5, 6, 7 };
 
         mesh.vertices = vertices;
         mesh.subMeshCount = 2;
         mesh.SetIndices(indexes1, MeshTopology.Quads, 0);
         mesh.SetIndices(indexes2, MeshTopology.Quads, 1);
     
         meshFilter.mesh = mesh;
 
         MeshRenderer renderer = (MeshRenderer)myObj.AddComponent(typeof(MeshRenderer));
         Material[] mat = new Material[2];
         mat[0] = new Material(Shader.Find("Standard"));
         mat[0].SetColor("_Color", Color.red);
         mat[1] = new Material(Shader.Find("Standard"));
         mat[1].SetColor("_Color", Color.blue);
         renderer.materials = mat;
 
         meshFilter.sharedMesh.RecalculateBounds();
         meshFilter.sharedMesh.RecalculateNormals();