To reproduce: please refer to the attached sample project and try to reorder a node. You will notice that the message box is not rendered properly.

        //case 2
        private void DragDropService_PreviewDragDrop(object sender, RadDropEventArgs e)
        {
             e.Handled = true;
            RadMessageBox.Show(this, "Showing a messagebox under dragending doesn't work correctly.");
        }

        //case 1
        private void radTreeView1_DragEnding(object sender, Telerik.WinControls.UI.RadTreeViewDragCancelEventArgs e)
        {
            //e.Cancel = true;
            //RadMessageBox.Show(this, "Showing a messagebox under dragending doesn't work correctly.");
        }

Workaround: subscribe to the TreeViewElement.DragDropService.PreviewDragDrop and set the Handled argument to true if you want to cancel the drop operation. Then, handle the TreeViewElement.DragDropService.Stopped event and show the desired message.

        public Form1()
        {
            InitializeComponent();
            
            this.radTreeView1.TreeViewElement.DragDropService.PreviewDragDrop += DragDropService_PreviewDragDrop;
            this.radTreeView1.TreeViewElement.DragDropService.Stopped+=DragDropService_Stopped;
        }

        private void DragDropService_Stopped(object sender, EventArgs e)
        {
            
            RadMessageBox.Show(this, "Showing a messagebox under dragending doesn't work correctly.");
        }
 
        private void DragDropService_PreviewDragDrop(object sender, RadDropEventArgs e)
        {
            e.Handled = true;
        }