Text does not appear in the Header and Footer areas of an XLSX export
If you setup a report header/footer which uses anything other than images (company logo, client logo, image) they will not be added to the header/footer cells in the XLSX doc. It will however add them to the print of the XLSX document. So when the document is printed a header and footer will be added to each page.
The request is text elements are included to be visible on the XLSX document when exported, and not just when printed.