I got 0.00Bytes file size indicated on attached XML file from mPDF library, and the document doesn’t confirm PDF/A-3

I use mPDF library to generate PDF/A-3 file, everything work fine except

1. my pdf file output from this library indicate file to only conform with PDF/A-1B.
2. my XML attachment in pdf document’s output indicate to be 0.00Bytes although I can open attached file and see detail.

How to fix this issue? Here is some of my code.

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$defaultConfig = (new MpdfConfigConfigVariables())->getDefaults();
$fontDirs = $defaultConfig['fontDir'];

$defaultFontConfig = (new MpdfConfigFontVariables())->getDefaults();
$fontData = $defaultFontConfig['fontdata'];

$mpdf = new MpdfMpdf([
  'format' => [210, 297]
  , 'mirrorMargins' => 1
  , 'mode' => 'utf-8'
    , 'PDFA' => true
  , 'PDFAauto' => false
  , 'PDFAversion'=> 'A-3'
  , 'fontDir' => array_merge($fontDirs, [
    $_SERVER['DOCUMENT_ROOT'] . '/font',
  ])
  , 'fontdata' => $fontData + [ // lowercase letters only in font key
    'sarabun' => [
        'R' => 'THSarabunNew.ttf'
        , 'B' => 'THSarabunNew Bold.ttf'
    ]
  ]
  , 'default_font' => 'sarabun'
  , 'debug' => true
]);


$mpdf->SetAssociatedFiles([[
    'name' => $invoiceNumber . '.xml'
  , 'mime' => 'text/xml'
  , 'description' => 'Tax Invoice XML Data'
  , 'AFRelationship' => 'Alternative'
  , 'content' => $xml
]]);

$mpdf->SetAdditionalXmpRdf($rdf);

$mpdf->SetTitle($invoiceNumber);
$mpdf->SetAuthor($masterCompany['companyName']);
$mpdf->SetCreator($masterCompany['companyName']);
$mpdf->SetSubject('Receipt/Tax Invoice');
$mpdf->SetKeywords('Tax Invoice');
$mpdf->WriteHTML($html);
$mpdf->Output($invoiceNumber . '.pdf', 'I');

I can’t find much examples in detail how to output PDF/A-3 document from mPDF from the internet. Here is what I got from there website https://mpdf.github.io/what-else-can-i-do/pdf-a3-xmp-rdf.html