To reproduce:

1. Open a RadForm
2. Maximize the form.
3. Restore the form. Then we can see odd line on the bottom of title bar / top of the form.  If you resize the form (make it bigger), new visible areas of form doesn't contain this weird line.

Workaround:
protected override void WndProc(ref Message m)
{
    FormWindowState currentWindowState = this.WindowState;
    base.WndProc(ref m);
    if (currentWindowState == FormWindowState.Maximized && this.WindowState== FormWindowState.Normal)
    {
      
        this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.SizableToolWindow;
        this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.Sizable;
    }
}