Hello,
phpdocx uses internal styles to be able to add new contents correctly. These styles are set in the base template (templates/phpdocxBaseTemplate.docx in Basic, Advanced and Premium licenses) and in the DOCXStructureTemplate.php file in the Premium license.
When using a custom template, some styles are imported. These default styles are available in OOXMLResources.php, $defaultPHPDOCXStyles variable. This is a static public variable so you can overwrite to set the styles to be imported (for example a set of them or none of them):
OOXMLResources::$PHPDOCXStyles = '<w:styles xmlns:w="http://schemas.openxmlformats.org/wordprocessingml/2006/main"/>';
$docx = new CreateDocxFromTemplate('document.docx');
You may need to use custom styles from your DOCX for new contents if needed (such as headings).
Regards.