You may find attached a test project where your scenario works as expected.
Some changes that I did to your scenario for it to work:
1. Added ContentControl to the RadTreeViewItemControlTemplate for the TreeView headers to be visualized.
2. I extended the MinHeight to 20pixels in the RadTreeViewItem style for better visibility of the Border you want to see when Selecting and moving away the Focus from the TreeViewItems
3. A sample TextBox control to grab the Focus from your control, so that you can observe the desired MultiTrigger change.
Could you please clarify that this is what you intended? If not - please give us steps to reproduce the unwanted behavior in the same project.
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