The text set to the custom dropdown buttons added to the FileExplorer's Toolbar is not visible in LightWeight rendering, unless the tooltips of the buttons are also configured.

Workaround (set the tooltip of the added button):

            RadToolBarDropDown dropDown = new RadToolBarDropDown("PageSize");
            dropDown.Text = "Page Size";
            dropDown.ToolTip = "Page Size";
            RadFileExplorer1.ToolBar.Items.Add(dropDown);