Since I am not able to reproduce the described issue, I would suggest reviewing the following help article
that explains in details about the RadPanelBar item's different expand modes. As the provided code snippet contains only one parent item with ExpandMode
set to FullExpandedItem,
clicking on that item could not collapse its child items. If you want to be able to expand and collapse its child items, I would suggest to try some other expand mode options that might suit better your scenario.
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