Issue
The automated snapshot backup retention period of RDS is configured, but backups are not deleted after the set retention period. The instance is scheduled to be automatically stopped during a specific time each day.
Cause
RDS does not count the time an instance spends in a stopped state when calculating the backup retention period.
Automated backups are not created while the DB instance is stopped. If the DB instance is stopped, backups may be retained longer than the configured retention period. RDS excludes the time spent in the stopped state from the backup retention period calculation.
Resolution
Adjust the retention period settings if necessary, considering that the time and state during which the DB instance is stopped are not included in the backup retention period.
Reference