I'm getting a strange issue with Graphics.DrawMeshNow(). As I change the camera angle, it seems as if it switches to another material from elsewhere in my scene, instead of using the material I am setting the pass of. My code is below:

//Variables - initialisation code not shown
Material mat; 
Mesh mesh; 
Vector3 pos; 
Quaternion rot; 
Vector3 scale; 

private void OnRenderObject() 
{ 
   for (int pass = 0; pass < mat.passCount; pass++) 
   { 
      mat.SetPass(pass); 

      Graphics.DrawMeshNow(mesh, Matrix4x4.TRS(pos, rot, scale)); 
   } 
}

What am I doing wrong (or not doing)?

Edit: A sample project that reproduces the issue can be downloaded here: Download