LibreOffice vVersion: 6.0.3.2 (x64) on a Windows 10 PC.
Each time I set the desired tabs in header and footer to one 6.5 inch right-aligned tab in each header and each footer (four total as a mirrored layout is used), correct tab persists in that session.
However close the document and reopen it in LibreOffice Writer, and the tabs in each header and footer have reverted to 3.46" center, 6.5" right or center, and 6.93" right. Am using the .docx file format.