If the height of RadComboBox is set to a value smaller than 26px (in Office2013) then the content of the RadToggleButton presenting the selected content is clipped.
This was reproduced with the Office2013 theme.
To work this around set the MinHeight of the child RadToggleButton to the same MinHeight as RadComboBox.
private
void
RadComboBox_Loaded(
object
sender, RoutedEventArgs e)
{
var comboBox = (RadComboBox)sender;
var toggle = comboBox.FindChildByType<RadToggleButton>();
toggle.MinHeight = comboBox.MinHeight;
}