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 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 unsynchronized backup, download it from the RDS console. To obtain the last backup, contact customer service.