To reproduce: 

- Add items with the same display member open the drop-down and select the second.

- Select another control on the form and then reopen the popup.

- The first item is selected.

Workaround: 

class MyMultiColumnComboBox : RadMultiColumnComboBox
{
    protected override void OnLostFocus(EventArgs e)
    {
        if (this.DropDownStyle == Telerik.WinControls.RadDropDownStyle.DropDownList)
        {
            return;
        }
        base.OnLostFocus(e);
    }
}