I figured out that the reason why the Klaviyo Flow is not working is because the Serial Key Tracking App, immediately marks a paid order as Fulfilled, then later adds the serial number from the list. So once it's fulfilled, it immediately triggers a a webhook/update to send out a serial serial number and hence it is blank.

As per the LicenseSpring Module, there should be a delay in between the time the order is marked as paid and the time in between an order is given a serial. See attached diagrams below. Once an order is paid, it should be marked as Unfulfilled  and it goes through an In Progress state, where there is a waiting period before a serial is issued. In our case, our app should verify that there are serials available that have not been given out. There may be an issue when the inventory count is off and it shows quantity available when it's not.Then and only then is a serial issued and the item is marked as Fulfilled.

Each product produced by the Serial Live Tracking App should produce separate line items for each product or order and not combine multiple serials for the same product.