Wrong selection in ListBox with VirtualizationMode="Recycling" and SeclectionMode="Extended"? - wpf

Question

I have a really strage behaviour. I have a ListBox in the View with a DataTemplate for its items including ViewModels. I bind the IsSelected to my ViewModel and use SelectionMode="Extended". Everything works fine.
BUT if I add VirtualiuationMode="Recycling" the I get the wrong items.
To reproduce: select items with Ctrl, then scroll down and select just one item. The normal behaviour deselects all items and just select the last one without holded Ctrl.
But if I check my ViewModel all the old items are selected!?!
<Grid>
<StackPanel>
<ListBox ItemsSource="{Binding People}" MaxHeight="100"
SelectionMode="Extended"
VirtualizationMode="Recycling">
<!--VirtualizingStackPanel.IsVirtualizing="True">-->
<ListBox.ItemContainerStyle>
<Style TargetType="{x:Type ListBoxItem}">
<Setter Property="IsSelected" Value="{Binding Path=IsSelected, Mode=TwoWay}" />
</Style>
</ListBox.ItemContainerStyle>
<ListBox.ItemTemplate>
<DataTemplate>
<views:PeopleView />
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
<Button Click="Button_Click">
OK
</Button>
</StackPanel>
</Grid>
The item template
<UserControl x:Class="WpfApplication1.View.PeopleView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Height="Auto" Width="Auto">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" SharedSizeGroup="A"/>
<ColumnDefinition Width="Auto" SharedSizeGroup="B"/>
</Grid.ColumnDefinitions>
<TextBox Text="{Binding Path=Name}"
Name="tbx_Name"
Grid.Column="0"/>
<CheckBox IsChecked="{Binding Path=IstAktiv}"
Name="cbx_IstAktiv"
Grid.Column="1"/>
</Grid>
Any idea?

Answer 1

I got a workaround but why do I have to change it "manually" in the change event and not by databinding?
private void Lbx_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
ListBox lbx = (ListBox)sender;
foreach (PersonViewModel item in lbx.Items)
{
item.IsSelected = lbx.SelectedItems.Contains(item);
}
}

Answer 2

Another option related to KCT's earlier answer is to use the AddedItems and RemovedItems from the SelectionChangedEventArgs and target the changes, such as:
private void Lbx_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
foreach (var item in e.AddedItems)
{
((PersonViewModel)item).IsSelected = true;
}
foreach (var item in e.RemovedItems)
{
((PersonViewModel)item).IsSelected = false;
}
}
This may give better performance with larger collections (I've got about 15,000 entries in a Virtualizing Tile Panel in a ListBox).