Why new discount codes are created every time?

If you have configured discounts on Frequently Bought Together, they will be created and applied automatically for convenience.

In order to avoid fraud, each discount code can only be used only once and only with its related bundle, i.e., the discount only applies to a particular combination of products.

For that reason, a new discount code is created every time a customer hits the 'Add to cart' button in the app widget.

You can check all created discounts in your Shopify Admin, section 'Discount', when they were created and if they were applied on checkout: