Vertex color emptied after instanciating mesh at runtime (build only)

It definitely could be a bug. Are you using GPU instancing? Also, when you modify the vertex colours, are you doing it post-instantiation (with different vertex colours per instance), or are the vertex colours shared between all copies of the mesh?

Example parts

Here's an example, just a single script and a basic model with vertex colours, the whole project was a bit big to upload, I've just put this together quickly to demonstrate the scenario. You'll notice it works fine in debug, but when you build it, it throws an out of bounds exception because the mesh.colors is empty.

Note: you need to turn on Read/Write Enabled on the mesh

Here's the code again to make it more visible:

  private void StretchMesh()
     {
         var instance = Instantiate(MeshObject);
         var meshes = instance.GetComponentsInChildren<MeshFilter>().Select(v => v.mesh);
 
         var moveDist = 10f;
         foreach (var mesh in meshes)
         {
             var verts = mesh.vertices;
             var colors = mesh.colors;
 
             for (var i = 0; i < verts.Length; i++)
             {
                 // On build this throws an index out of range exception because the colours are empty
                 verts[i].x += moveDist * colors[i].r;
             }
 
             var newMesh = new Mesh
             {
                 vertices = verts,
                 triangles = mesh.triangles,
                 colors = mesh.colors,
                 normals = mesh.normals,
                 tangents = mesh.tangents,
                 uv = mesh.uv,
             };
             newMesh.RecalculateBounds();
 
             var go = new GameObject("Test");
             var filter = go.AddComponent<MeshFilter>();
             var renderer = go.AddComponent<MeshRenderer>();
             filter.mesh = newMesh;
             renderer.material = TestMaterial;
         }
     }