HikaSerial PDFSerial ticket not attached to order status email

-- HikaShop version -- : 6.6.0
-- HikaSerial version -- : 6.1.1
-- Joomla version -- : 5.4.7
-- PHP version -- : 8.3
-- Browser(s) name and version -- : Firefox 155.0.1
-- Error-message(debug-mod must be tuned on) -- : No message

Hello,

we are using:

HikaShop 6.6.0
HikaSerial Standard 6.1.1
HikaShop PDF Invoice plugin 4.1.0

We use HikaSerial PDFSerial to generate PDF tickets for event registrations.

The serial is correctly assigned to the ordered product and is displayed in the customer order status email.

PDFSerial itself also works correctly:

the PDF ticket is generated correctly
the virtual file #hikaserial:pdfserial:1 works
the customer can download/open the generated PDF ticket

However, the PDF ticket is not attached to the order status email, although Attach to emails is enabled in the PDFSerial configuration and the correct serial pack is selected.

We already tested the Joomla PDFSerial plugin settings with both:

Attach a virtual file: Yes / No
Add attachment with mime type: Yes / No

The result is always the same: the HikaShop invoice PDF is attached to the email, but the PDFSerial ticket is not.

The HikaSerial configuration is set to display serials with both statuses Assigned and Used.

During mail sending we also get this PHP warning:

PHP Warning: Constant PDF_FONT_NAME_MAIN already defined in plugins/hikashop/attachinvoice/vendor/spipu/html2pdf/src/config/tcpdf.config.php on line 209

Could there be a compatibility issue between PDFSerial and the HikaShop PDF Invoice plugin / TCPDF?

We also checked the HikaSerial 6.1.1 code. pdfserial.php attaches the ticket in onBeforeSerialMailSend(), while the HikaShop/HikaSerial integration calls beforeMailSend() for order mails.

For testing, we added temporary logging to onBeforeSerialMailSend(), but when sending the order status email we did not see the method being reached.

Is there any additional configuration required for PDFSerial email attachments with HikaShop 6.6.0 / HikaSerial 6.1.1, or is this a known issue?

Thank you.