Actually it is not a bug. It is because the background or fill brush of the scrollable element is null. Null fill and background brushes allow mouse events to go through to whatever is behind the element in question.

Instead, you can set a fill or background brush to the predefined color “Transparent”, and your element will now always intercept mouse and touch actions and raise the corresponding events.

Example:

<Intersoft:UXScrollViewer Name="SampleControl1" Intersoft:DockPanel.IsFillElement="True">
    <Intersoft:UXItemsControl BorderThickness="0"
                        ItemsSource="{Binding Path=FilteredBooks}" Background="Transparent">
        ...
    </Intersoft:UXItemsControl>
</Intersoft:UXScrollViewer>

Hope this help.