Hello,
As a textbox also uses a drawing element, you need to select the textbox tag in the query:
$referenceNode = array(
'customQuery' => '//w:txbxContent//w:drawing[descendant::wp:docPr[contains(@descr, "$IMAGE$")]]',
);
$docx->removeWordContent($referenceNode);
//w:txbxContent has been added to the query.
If you send a sample DOCX document you are using to contact[at]phpdocx.com we can generate a custom script using it.
Regards.