Hi,
Please be sure that your custom field does not have "input filtering" activated, this option strip the HTML tags.
Otherwise, there is a fix for HikaMarket but regarding your configuration, I don't think it's linked.
You can edit the file "administrator/components/com_hikamarket/classes/product.php" and replace the line
$product = $fieldsClass->getInput('product', $oldProduct);
By
$product = $fieldsClass->getInput('product', $oldProduct, true, 'data', false, 'all');
It will fix the loading of the custom field when not visible in the front-end.
Regards,