How to reproduce: check the attached project

Workaround: resize the size of the hosting RadPanel
private void radCollapsiblePanel_Expanding(object sender, CancelEventArgs e)
{
    this.radPanel1.Size = new Size(this.radPanel1.Size.Width, this.radPanel1.Size.Height - 1);
}

private void radCollapsiblePanel_Expanded(object sender, EventArgs e)
{
    this.radPanel1.Size = new Size(this.radPanel1.Size.Width, this.radPanel1.Size.Height + 1);
}