To reproduce:
- Zoom the diagram and drop an item from the toolbox. 

Workaround:
Friend Class MyToolbox
    Inherits RadDiagramToolbox
  
    Protected Overrides Sub DragDropService_PreviewDragDrop(ByVal sender As Object, ByVal e As RadDropEventArgs)
        'base.DragDropService_PreviewDragDrop(sender, e);
        Dim shape = TryCast(GetType(RadDiagramToolbox).GetField("shape", System.Reflection.BindingFlags.NonPublic Or System.Reflection.BindingFlags.Instance).GetValue(Me), RadDiagramShape)
  
        Dim diagramElement As RadDiagramElement = TryCast(e.HitTarget, RadDiagramElement)
        Dim containerShape As RadDiagramContainerShape = TryCast(e.HitTarget, RadDiagramContainerShape)
        If containerShape IsNot Nothing Then
            diagramElement = containerShape.FindAncestor(Of RadDiagramElement)()
        End If
  
        If diagramElement Is Nothing OrElse diagramElement.Shape IsNot Nothing OrElse shape Is Nothing Then
            Return
        End If
  
        e.Handled = True
        Dim zoom As Double = diagramElement.Zoom
        Dim m As Telerik.WinControls.Layouts.RadMatrix = diagramElement.MainPanel.Transform
        m.Invert()
  
        Dim position = New Telerik.Windows.Diagrams.Core.Point(m.DX + (e.DropLocation.X - shape.Width \ 2) * 1 / zoom, m.DY + (e.DropLocation.Y - shape.Height \ 2) * 1 / zoom)
        If containerShape IsNot Nothing Then
            position.X += containerShape.Position.X
            position.Y += containerShape.Position.Y
        End If
  
        shape.Position = position
        diagramElement.AddShape(shape, Nothing, True)
        If containerShape IsNot Nothing Then
            DirectCast(containerShape, IContainerShape).AddItem(shape)
        End If
  
        shape = Nothing
    End Sub
End Class