Hello,
What version of LibreOffice are you using? We have done some tests and keep next option is working correctly when transforming DOCX to PDF. If you send to contact[at]phpdocx.com the most simple DOCX that illustrate your issue we'll check it.
Thanks for the warning about the documentation issue on the addHeading page, we have removed the wrong default value comment on the documentation page. Option types are being checked.
Regards.