How to reproduce: 

public Form1()
{
    InitializeComponent();

    new RadControlSpyForm().Show();

    for (int i = 0; i < 2; i++)
    {
        this.radCheckedDropDownList1.Items.Add("Item &" + i);
    }

    this.radCheckedDropDownList1.VisualListItemFormatting += radCheckedDropDownList1_VisualListItemFormatting;
}



Workaround:

private void radCheckedDropDownList1_VisualListItemFormatting(object sender, VisualItemFormattingEventArgs args)
{
    RadCheckedListVisualItem item = args.VisualItem as RadCheckedListVisualItem;
    RadLabelElement lbl = item.FindDescendant<RadLabelElement>();
    lbl.UseMnemonic = false;
}