Hi,
The cart is emptied when the order is created. There is no reason it wouldn't.
It might be because you have previous carts for your current user (due to your repeated tests) and thus the system loads the next one in line.
Go in the menu Customers>Carts, clear all the carts and try again.
You can see that in Authorize.net plugin (file plugins/hikashoppayment/authorize/authorize.php) you have that code which clears the session after the order is created:
$class = hikashop_get('class.cart');
$class->cleanCartFromSession();
And if you look at that function (file administrator/components/com_hikashop/classes/cart.php) you have that code:
$app->setUserState( HIKASHOP_COMPONENT.'.cc_number', '');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_month', '');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_year', '');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_CCV', '');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_type', '');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_owner', '');
$app->setUserState( HIKASHOP_COMPONENT.'.cc_valid', 0);
which wipes out the credit card data of the user session.
So I don't see why you would still get the credit card data in the user session after the order is created, unless you have a problem with your server's PHP user session system. You might want to try changing the "session handler" setting of the Joomla configuration, and/or try on a different server to see if you can replicate the problem. On my end, I can't reproduce the problem.