When you undock a RadPane from the RadDockPanel it is removed from the visual tree, this may cause an ElementName bindings to fail. As you are using the TitleTemplate of the RadPane its DataContext comes from its Title property. If you want to access a property from your ViewModel inside your custom TitleTemplate you will need to set to the Title property of the pane the ViewModel responsible for the commands in the title. Binding your ViewModels exclude the need to use an ElementName binding.
I have attached a sample project demonstrating the above approach.
I hope this information helps.
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