Hi,
The value of custom order fields is stored along the cart data in the database until it is stored with the order at the end of the checkout when the order is created.
So it's normal that the new default value is not seen for users with already existing carts.
The data of the custom order fields is stored in the cart_fields column of the hikashop_cart table.
So an easy solution would be to run a MySQL query to clear that field for all the carts via your PHPMyAdmin:
UPDATE #__hikashop_cart SET cart_fields = '';
where #__ is to be replaced with the table prefix from your Joomla configuration.
However, that means that the data of all the custom order fields of all the carts not yet converted into orders will be lost. You can't just change the value of one field for all the carts.