Google Merchant export issue with FALSE and MPN turned ON

-- HikaShop version -- : 4.0.3
-- Joomla version -- : 3.9
-- PHP version -- : 7.3

Hi Hikashop team,

There is an issue with the xml export.

If you turn on the Add Product Code (mpn) field.
Then all the products without a GTIN and with the setting to FALSE (through custom fields) also get a MPN code.

But google don't like this, see error below:
When product identifiers are provided 'identifier_exists' should be set to true or not set at all. Setting identifier_exists to false when product identifiers actually exist might impact ranking of an item in the future.

Would it be an idea for this situation to have a possibility to specify a separate product custom field for MPN and don't use the product code.

Kind regards,
Lumiga