Workaround:
            ContextMenuService menuService = this.radDock1.GetService<ContextMenuService>();
            menuService.ContextMenuDisplaying += menuService_ContextMenuDisplaying;

        private void menuService_ContextMenuDisplaying(object sender, ContextMenuDisplayingEventArgs e)
        {
            foreach (RadMenuItem listMenuItem in e.MenuItems)
            {
                TextPrimitive textPrimitive = listMenuItem.Layout.TextPanel.Children[0] as TextPrimitive;
                textPrimitive.UseMnemonic = false;
            }
        }