We recently diagnosed a problem with our expiration mailer that was causing it to crash frequently, meaning a number of expiration emails did not go out when they should have. We deployed a hotfix for the problem today at 2300 UTC.
The buggy code was first deployed on June 16th, and we send out the first expiration warning emails 20 days in advance of expiration. So all subscribers should still get warning of expiration, but some will receive warning later than expected.
We'll be developer better monitoring to prevent this from happening in the future.