You may notice that your billing was changed from monthly to annual. Performing any changes to it between March 7th and May 9th 2021 may have caused changes in the expiration date of your messages.
Up to March 7th 2021, billing was always on a monthly basis, regardless of the text expiration.
Between March 7th and May 9th 2021, billing and message expiration were combined; monthly billing meant monthly expiration, etc.
As of May 9th 2021, the Billing Plan Period and Credits Rollover Period are configured separately. It is advisable that you check your existing plans to verify that everything is configured the way it should be. You can adjust your current purchasing plans to reflect correct billing and expiration periods as described in this article.