Large triangular polygon with a large number of points in the mesh

Hello everyone! When creating the mesh, I had a problem: both in the game, and in the scene appears a large triangular polygon. The problem is that with a system size of 250 by 250 points there aren't this polygon. At a size of 400 by 400 points, it already appears.

What I did:

 void Start () {
             MeshFilter mf = GetComponent<MeshFilter>();
             Mesh mesh = mf.mesh;
             int size = 400;
             Vector3[] vertices = new Vector3[size * size];
             Vector3[] normals = new Vector3[size * size];
             Vector2[] uvc = new Vector2[size * size];
     
             int c = 0;
             for (int i =0; i < size; i++)
             {
                 for(int j =0; j < size; j++)
                 {
                     vertices[c] = new Vector3(i, j, Mathf.Sin(j+i+Mathf.PI));
                     normals[c] = -Vector3.forward;
                     float mult = 1.0f / ((float)(size));
                     uvc[c] = new Vector2(((float)(i))*mult, ((float)(j)) * mult);
                     c++;
                 }
             };
     
             int[] triangles = new int[(size-1)*(size-1)*6];
             int counter = 0;
             
             {
                 for (int i = 0; i < size - 1; i++)
                 {
                     for (int j = 0; j < size - 1; j++)
                     {
                         setTriangle(ref triangles, ref counter, i * size + j);
                         setTriangle(ref triangles, ref counter, (i + 1) * size + j);
                         setTriangle(ref triangles, ref counter, i * size + j + 1);
     
                         setTriangle(ref triangles, ref counter, (i + 1) * size + j);
                         setTriangle(ref triangles, ref counter, (i + 1) * size + j + 1);
                         setTriangle(ref triangles, ref counter, i * size + j + 1);
     
                     };
                 }
             };
     
             mesh.Clear();
             mesh.vertices = vertices;
             mesh.triangles = triangles;
             mesh.normals = normals;
             mesh.uv = uvc;
             mesh.RecalculateNormals();
     
             gameObject.transform.position = new Vector3(-55, -60, 90);
     
         void setTriangle(ref int[] triangle, ref int index, int num)
         {
             triangle[index++] = num;
         }
 
 

And what I have in results. The first picture is with 250 by 250 points. The second is with 400 by 400. The third is just bigger picture.alt text