Workaround:

Sub New()

    InitializeComponent()
     
    Me.RadScheduler1.ActiveViewType = UI.SchedulerViewType.Week
    AddHandler Me.RadSchedulerNavigator1.SchedulerNavigatorElement.ShowWeekendCheckBox.ToggleStateChanged, AddressOf ToggleStateChanged
End Sub

Private Sub ToggleStateChanged(sender As Object, args As UI.StateChangedEventArgs) 
    Me.RadScheduler1.SchedulerElement.RefreshViewElement()
End Sub