To reproduce: please refer to the attached sample project and gif file illustrating the visual problem.

Workaround:  subscribe to the RadSchedulerNavigator.ShowWeekendStateChanged event and refresh the scheduler view element:

AddHandler Me.RadSchedulerNavigator1.ShowWeekendStateChanged, AddressOf ShowWeekendStateChanged

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