Hi Rafael, thanks for reaching out. This is correct - if you have a monthly subscription, unused credits will roll over and accumulate in your account up to 2x the credit limit according to your plan. If you buy an annual plan, you get all your credits upfront, and you can use them however and whenever you’d like.
You can read more about the Credit Roll Over Update here. If you have massive amounts of unused credits, you can reach out to us and we can determine wether we can manually roll over some of those credits for you on a case-by-case basis.
I hope this was helpful. If there's anything else I can assist you with, please let me know!