To reproduce:
- Add to collapsible panels to a form.
- Dock the first to the top and set the Dock property of the second to Fill.
- Set the ExpandDirection of the second to Up.
- Start the application and collapse and then expand the second panel.
- You will notice that the header button is no longer visible.

Workaround:
void radCollapsiblePanel1_Expanded(object sender, EventArgs e)
{
    radCollapsiblePanel2.CollapsiblePanelElement.InvalidateMeasure(true);
}