Hello,
We have checked your code and it's working as expected.
Please note that the current version of phpdocx add cross-references as PAGEREF (not bookmark text content), so if you update its value manually (MS Word, right click, Update field) or automatically (using a macro or ) the content shows the page number of the linked bookmark not the bookmark text content.
To add an internal link that displays a text content, you can use addLink (phpdocx_link using HTML Extended) with the bookmark name.
We move the request of applying the bookmark text content to the dev team to be added to the next release of phpdocx.
Regards.