Hello Kevin Kutlesa,
This is expected behavior of the RadTreeView. It loads its items when they are root level items and they are in a "viewable" area of the application or when they are non-root and their parent is expanded. In your case, if the RadDropDownButton is not expanded, the Items are not loaded and the CheckedItems collection is empty.
As a workaround, I suggest you to use databinding and implement your own CheckedItems collection in your ViewModels. As a good starting point, you can check out this project demonstrating how to implement your own "tri-state" logic in the ViewModels.
All the best,
the Telerik team
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