To reproduce:
- Add pie chart and drop down list with some items to a form.
- Then use the following code:

Private Sub RadForm1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Dim series As New PieSeries

    series.DataPoints.Add(New PieDataPoint(50, "test"))
    series.DataPoints.Add(New PieDataPoint(50, "test2"))

    series.ShowLabels = True

    RadChartView1.Series.Add(series)

    TryCast(RadChartView1.Series(0).Children(0), PiePointElement).BackColor = Color.Red
    TryCast(RadChartView1.Series(0).Children(1), PiePointElement).BackColor = Color.Blue
End Sub

Private Sub RadChartView1_LabelFormatting(sender As Object, e As ChartViewLabelFormattingEventArgs) Handles RadChartView1.LabelFormatting
    e.LabelElement.Shape = New OfficeShape(True)
End Sub

Workaround:
e.LabelElement.SuspendPropertyNotifications()
e.LabelElement.Shape = New OfficeShape(True)
e.LabelElement.ResumePropertyNotifications()