The issue reproduces if you set the SelectedItem of the RadMultiColumnComboBox control before its textbox is loaded. One way for this situation to occur is if you data bind the SelectedItem to a property of the view model and then assign to the view model to the DataContext of the view, before the InitializeComponent call.
To work this around, you can reset the SelectedItem property on Loaded.
private void RadMultiColumnComboBox_Loaded(object sender, RoutedEventArgs e)
{
var mccb = (RadMultiColumnComboBox)sender;
var selection = mccb.SelectedItem;
mccb.SetCurrentValue(RadMultiColumnComboBox.SelectedItemProperty, null);
mccb.SetCurrentValue(RadMultiColumnComboBox.SelectedItemProperty, selection);
}