Hello,
As explained on the documentation pages, phpdocx requires adding UTF8 contents. If non UTF8 contents are added, the library tries to transform them to UTF8 using mb_ functions from PHP.
It seems your content can't be detected automatically to be transformed to UTF8 (maybe it's mixing charsets). You can convert them to UTF8 using utf8_encode from PHP before adding it:
$html_encoded = utf8_encode($html);
$docx->embedHTML($html_encoded);
Regards.