Binded items in TabControl don't display after switching template

[tms320]

I have hierarchy of view models:

class MainViewModel
{
    ObservableCollection<NetworkViewModel> Networks { get; }
}

class NetworkViewModel
{
    ObservableCollection<NodeViewModel> Nodes { get; }
}

I need to display list of Networks and list of Nodes inside each Network.
TabControl is of course suitable for this purpose. But I want to use it only if the number of Networks is greater than 1. Otherwise, I want to use ItemsControl, because I don't want to display any Tab Header for single network. So I created a ContentControl (named 'NetworksView', see below), whose template is selected by a selector depending on the number of Networks.
To display Nodes inside a Network, I use TabControl named 'PART_NodesTabControl' via template.
So here is my xaml code:

<Window.Resources>
    <DataTemplate x:Key="NodesDataTemplate">
	<TabControl Name="PART_NodesTabControl" ItemsSource="{Binding Nodes}"/>
    </DataTemplate>
</Window.Resources>

<ContentControl Name="NetworksView" Content="{Binding Networks}">
	<ContentControl.ContentTemplate>
		<vm:NetworksTemplateSelector>
			<DataTemplate x:Key="SingleNetworkTemplate">
				<ItemsControl ItemsSource="{Binding}" ItemTemplate="{StaticResource NodesDataTemplate}"/>
			</DataTemplate>
			<DataTemplate x:Key="MultipleNetworksTemplate">
				<TabControl ItemsSource="{Binding}" ContentTemplate="{StaticResource NodesDataTemplate}"/>
			</DataTemplate>
		</vm:NetworksTemplateSelector>
	</ContentControl.ContentTemplate>
</ContentControl>

The creation of view models and assignment of DataContext are performed in the window constructor (after InitializeComponent). At this point, the list of networks and nodes in them already exists.
Then everything is displayed normally. If there is more than one network, then a TabControl with a list of networks is displayed, and nodes of this network are displayed inside each tab. If there is only one network, then only a TabControl with a list of nodes of this network is displayed. I can find TabControl named 'PART_NodesTabControl' (see template above) in code behind and see that its ItemsSource.Count is equal to the number of Nodes.
The problem is: if the number of networks changes while the application is running (between 1 and 2, for example), then for some reason the TabControl with a list of nodes stops displaying. In this case PART_NodesTabControl.ItemsSource.Count is zero, even though ItemsSource is bound to the same valid source collection and this collection has a non-zero number of items.
I tried to re-set PART_NodesTabControl.ItemsSource to the same items source. It doesn't help.

[timunie]

Why not simply hide the TabHeader if Count is 1? You may need to copy the ControlTemplate to do so probably