Setting IsLinkedToPrevious to false is not respected on export to DOCX/RTF for an empty header when the footer contains page field. The header is not exported at all.

Workaround: Add some content to the header (it might be a single space).

foreach (var section in this.radRichTextBox.Document.Sections)
{
    if (section.Headers.Default.IsEmpty)
    {
        RadDocumentEditor editor = new RadDocumentEditor(section.Headers.Default.Body);
        editor.Insert(" ");
    }
}