Had also contact with WooCommerce Payments and it seems a known issue on there side..
https://github.com/Automattic/woocommerce-payments/issues/3749
with the comment: It’s a known issue and the reason why we added ‘… in supported devices here.’ The only solution to display flags in not supported devices will be creating a custom select element, but we (included design team) decided that it was not worth the effort.