Procedurally generated mesh having some faces that don't receive light

Hi,

I'm having some troubles using lights on a procedurally generated mesh. It seems that depending on the point light position, some faces will not receive light, even if they are fully exposed.

This is how I generate the mesh :

 using UnityEngine;
 using System.Collections;
 
 public class MeshGen : MonoBehaviour {
 
     public GameObject obj;
 
     void Start () {
         generate();
     }
 
     void generate(){
         Vector3 pt0 = new Vector3(0,0,0);
         Vector3 pt1 = new Vector3(0,1,0);
         Vector3 pt2 = new Vector3(1,1,0);
         Vector3 cross = GetNormal (pt0, pt1, pt2);
         Vector3 pt3 = pt0+cross;
         Vector3 pt4 = pt1+cross;
         Vector3 pt5 = pt2+cross;
         Vector3[] newVertices = {pt2, pt1, pt0, 
             pt3, pt4, pt5, 
             pt0, pt1, pt3, 
             pt1, pt4, pt3, 
             pt0, pt3, pt2, 
             pt2, pt3, pt5,
             pt1, pt2, pt4, 
             pt2, pt5, pt4};
         int[] newTriangles = {0,1,2,
             3,4,5,
             6,7,8,
             9,10,11,
             12,13,14,
             15,16,17,
             18,19,20,
             21,22,23
         };
         GameObject myFace = (GameObject) Instantiate(obj, new Vector3(0,0,0), Quaternion.identity);
         Mesh mesh = new Mesh();
         myFace.GetComponent<MeshFilter>().mesh = mesh;
         mesh.vertices = newVertices;
             Vector3[] vertices = mesh.vertices;
         Vector2[] uvs = new Vector2[vertices.Length];
         for (int i=0; i < uvs.Length; i++) {
             uvs[i] = new Vector2(vertices[i].x, vertices[i].z);
         }
         mesh.uv = uvs;
         mesh.triangles = newTriangles;
     }
 
     public static Vector3 GetNormal(Vector3 a, Vector3 b, Vector3 c) {
         Vector3 side1 = b - a;
         Vector3 side2 = c - a;
         return Vector3.Cross(side1, side2).normalized;
     }
 }

And this is the result (the generated mesh is on the left) :

Could someone tell me what I'm doing wrong please ?

Thanks a lot.