If the expiration date of a customer's subscription is in the past but it is still active, then that subscription is in a grace period.
For example, this user signed up for a 3-day free trial of the 1-month subscription on May 5, 2020 which should have converted on May 08, 2020. However, the dashboard shows the expiration date as May 24, 2020. This is because Apple applied a 16-day grace period due to billing issues. Apple will continue attempting to bill the user until they successfully do so or reach the end of the 16-day grace period, at which point you'll see a Billing issue event in the dashboard.