Yes, your credits purchased during and as part of a renewing subscription will no longer be eligible for carryover. If you cancel, any unused credits will expire 30 days after cancellation.
For example, if you purchase a subscription on April 2, 2023, for 100 credits a month and you use all 100 credits in April, but then do not use any of the May 2 or June 2 credits, the 200 credits you received in those 2 months will carry over for up to a year after the purchase date if you have an active subscription.
If you cancel your subscription on June 15, 2023, before your July renewal, you will need to use all 200 accumulated credits by 30 days after June 15.
They only continue to roll over when your subscription is current. If you wait to cancel your subscription after July 2nd, you would have another month to use the 300 credits you would now have in your account.