WPF ComboBox SelectedItem Set to Null on TabControl Switch

I've got a simple problem in my WPF application which has me banging my head on the table. I have a TabControl, where every TabItem is a View generated for a ViewModel using a DataTemplate similar to this:

<DataTemplate DataType="{x:Type vm:FooViewModel}">
    <vw:FooView/>
</DataTemplate>

FooView contains a ComboBox:

<ComboBox ItemsSource="{Binding Path=BarList}" DisplayMemberPath="Name" SelectedItem="{Binding Path=SelectedBar}"/>

and FooViewModel contains a simple Property: public Bar SelectedBar { get; set; }. My problem is that when I set the value for my ComboBox, change to another tab, then change back, the ComboBox is empty again. If I set a breakpoint on the setter for my property, I see that the property is assigned to null when I switch to another tab.

From what I understand, when a tab is switched, it is removed from the VisualTree - but why is it setting my ViewModel's property to null? This is making it very difficult for me to hold persistent state, and checking value != null does not seem like the right solution. Can anyone shed some like on this situation?

Edit: The call stack at the setter breakpoint only shows [External Code] - no hints there.