I have databound RadTreeViewItem.IsExpanded in my item container style, but when the item is created, it ignores the binding (which is set to true) and sets the property to false. I found another thread in the forum describing the same behavior happening with the IsChecked property.
Call stack:
Telerik.Windows.Controls.Navigation.dll!Telerik.Windows.Controls.RadTreeViewItem.IsExpanded.set(bool value) Line 390
C#
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Telerik.Windows.Controls.Navigation.dll!Telerik.Windows.Controls.RadTreeViewItem.OnIsExpandedChanged(bool oldValue, bool newValue) Line 1186
C#
Telerik.Windows.Controls.Navigation.dll!Telerik.Windows.Controls.RadTreeView.RestoreContainerProperties(object item, Telerik.Windows.Controls.RadTreeViewItem treeViewItem) Line 678
C#