How Are Unsynchronized Backups Generated for RDS SQL Server DB Instances?
Unsynchronized backups are generated only for Microsoft SQL Server 2017 Enterprise Edition DB instances. If a primary DB instance fails, the standby DB instance is promoted to be the new primary instance. During the failover process, a small amount of data may not be synchronized and a differential backup is created for user-created databases on the original primary DB instance. You can use the unsynchronized backup and the last backup to restore data.
To obtain the most recent backup, contact customer service.