The initial selection doesn't work when RadComboBox is placed in a DataTemplate (examples: ContentControl.ContentTemplate, GridViewDataColumn.CellTemplate) and the SelectedValuePath property of the control is set in a Style DataTrigger. To reproduce this the Binding of the DataTrigger should use a converter.

To work this around avoid using a binding in a DataTrigger to set the SelectedValuePath. Set the property directly on the RadComboBox instance, or use another technique based on your requirements.