Hi,
That's normal. The order_created is set by HikaShop inside the save function for new orders, regardless of what you provide.
What you can do is to resave the order with the order_created after the first save.
Otherwise, you can also implement the "onBeforeOrderCreate" event and set the order_created in there. The event happens after the save function sets the order_created with the current date, but before the order data is saved in the database. So it's a nice and lightweight method to force the order_created value to whatever you want.