I haver the same issue but never got an error during the update. I checked my overrides and nothing is there too.
Also checked my tables and mpn is there in all required tables.
Uncaught PHP Exception Symfony\Component\Debug\Exception\ContextErrorException: "Notice: Undefined property: Product::$mpn" at /home/xxxx/public_html/src/PrestaShopBundle/Model/Product/AdminModelAdapter.php line 656
Upgrading PS is truly a nightmare and should be a debug priority.