Please refer to the attached gif file and sample project. When you hide the overflow button with the first pixel, although it is indicated as Visible, it is actually not shown.

Workaround: Timer timer = new Timer();

private void commandBarStrip_ItemOverflowed(object sender, EventArgs e)
{
    timer.Interval = 100;
    timer.Tick += timer_Tick;
    timer.Start();

}

private void timer_Tick(object sender, EventArgs e)
{
    timer.Tick -= timer_Tick;
    var commandBarStrip = this.radPdfViewerNavigator1.CommandBarElement.Rows[0].Strips[0];
    commandBarStrip.OverflowButton.Visibility = ElementVisibility.Visible;
    timer.Stop();
}