The SelectionChanged event is fired before Page_Load, hence when the ViewState is applied, it overrides the changes made inside the OnSelectionChanged handler.

Example - Clearing the CheckBox selection of a RadComboBox inside SelectionChanged

Workaround - Create a custom boolean property on the page and set it to true inside the SelectionChanged. Then, in Page_Load or the OnLoad event of any control, execute your logic based on the custom boolean property value.