Hello,
Thank you for the report. You're right, this is a regression that slipped into 6.4.0. During an internal refactoring of how payment plugins record information on orders, the step that stores the beginning of the credit card number on the order stopped being executed. The rest of the flow was not affected, which is why the end of the number still reaches the shop owner through the email notification as before.
We have just fixed the issue and it has been included in the 6.4.1 version of HikaShop we just released.
If you need the fix without updating, you can edit the file administrator/components/com_hikashop/helpers/paymentplugin.php. Near the top of the modifyOrder function, locate this line:
if(!empty($id) && $id !== false) {
and change it to:
if(!empty($id) || is_object($order_id)) {
After saving the file, new orders will again show the beginning of the credit card number in the backend.