Hello,
Please note that PHP should add or show a warning or error when it exceeds some limit (memory, time...). Do you get some information in the server logs or stdout when doing the conversion?
As the same issue happens using PHP CLI mode, some external setting or program must be causing this strange issue (some antivirus or external program? or some missing permission (https://www.php.net/manual/es/ref.com.php#120122)?). Is the same problem repeated with another document? for example a very simple DOCX that contains only a text paragraph.
What happens if the code ends the PHP execution before releasing the PHP COM instance?:
exit;
$MSWordInstance->Quit();
$MSWordInstance = null;
What Windows, MS Word and PHP versions are you using? If you send to contact[at]phpdocx.com a DOCX sample you are transforming we'll test it in our Windows test server.
Regards.