I've tried on product purchase as well as on item purchase (based on your description) and everything works correctly.
Also keep in mind that invoice for admin payments and payment with credits will not generate invoice as these are not real payments.
- admin pay -> is not real payment, it's just for testing purpose or manual order creation
- credit payments -> you've paid for credits (generate invoice) or credits were assigned based on periodical bonus, registration ... and these are not real payments