Hello,
The problem of the documents of this user was that both DOCX had a default font, so the final document used the first default font for the whole document (a DOCX can't have more than one default font).
To solve this issue, instead of using:
$docx->setDefaultFont("Roboto");
you need to set the font family for each content, for example:
$docx->addText("Lorem ipsum", array('font' => 'Roboto'));
After doing this change all fonts merge fine.
Regards.