Because I had layout problems with the GDPR extension from J!Extensions Store, I asked a support question there.
The developer discovered a bug in the T4 template framework.
Please fix the code in the file public_html/plugins/system/t4/src/t4/Helper/Metadata.php
from:
$document->addCustomTag('<meta property="'.htmlspecialchars($name, ENT_COMPAT, 'UTF-8').'"'.$attributes.' content="' . htmlspecialchars($value, ENT_COMPAT, 'UTF-8') . '" />');
to:
if($document->getType() == 'html') {
$document->addCustomTag('<meta property="'.htmlspecialchars($name, ENT_COMPAT, 'UTF-8').'"'.$attributes.' content="' . htmlspecialchars($value, ENT_COMPAT, 'UTF-8') . '" />');
}
It looks like you haven't applied Joomla's document type here.