Custom checkout field does not show on PDF any more. (Before it worked fine.)

Hello

Until a few weeks ago we were able to show the VAT number of customers (= a custom checkout field) on the pdf invoices. We used this code to achieve that:

add_action( 'wpo_wcpdf_after_order_data', 'wpo_wcpdf_delivery_date', 10, 2 );
function wpo_wcpdf_delivery_date ($template_type, $order) {
    if ($template_type == 'invoice') {
        $document = wcpdf_get_document( $template_type, $order );
        ?>      
        <tr class="delivery-date">
            <th>Ondernemingsnr:</th>
            <td><?php $document->custom_field('billing_wooccm13'); ?></td>
        </tr>
        <?php
    }
}

For some reason the field “billing_woocm13” does not show up any more. The label of that field (“Ondernemingsnr:”) is still on the invoice. So that works. Only the content of that custom field is missing …

(For testing purposes I replaced the code <?php $document->custom_field(‘billing_wooccm13’); ?> with the word TEST and then it shows “TEST” on the PDF invoice … )

What can be the reason and how do we solve this?

Thank you for the support!