WPF's BitmapCache is not completely cleared

We have a WPF Page with a user control where we use a BitmapCache - when we try to clear this element by updating the property (Data Binding) with an empty Path (New Path()), it is not completely refreshed/cleared. If I change the window size a little bit the area where the BitmapCache is active is completely cleared.

Is there something special to do to clear/refresh an element that uses BitmapCache?

This is our code:

    <me:ScrollViewer
    RenderedWaves="{Binding RenderedWaves}"
    ItemTemplate="{DynamicResource DataTemplateForWaveItem}" 
    ItemsPanel="{DynamicResource ItemsPanelTemplateForWaveItems}" 
    CacheMode="BitmapCache" />

I thought I fixed it, but it works not every time...

This code to set the path doesn't update the BitmapCache immediately:

Protected WriteOnly Property SetGraph As Path
 Set(value As Path)
    If value Is Nothing Then value = GetEmptyPath()
    _graph = value
    OnPropertyChanged(New PropertyChangedEventArgs(PropertyNameGraph))
 End Set
End Property

And this code updates it sometimes:

Protected WriteOnly Property SetGraph As Path
Set(value As Path)
    UIDispatcherLocator.UIDispatcher.Invoke(Sub()
                                                If value Is Nothing Then value = GetEmptyPath()
                                                _graph = value
                                            End Sub, Threading.DispatcherPriority.Background)
    OnPropertyChanged(New PropertyChangedEventArgs(PropertyNameGraph))
End Set
End Property