Cross-Region RDS Replica Not Syncing Existing Data

When you create a cross-region read replica for an RDS instance, the existing data from the master should typically be transferred to the replica during the initial setup. The fact that this didn't happen in your case is unusual and suggests there might be an issue with the replication process or configuration.

Here are a few potential reasons and things to check:

1. Replication lag: There might be a significant replication lag due to the cross-region nature of your setup. Cross-region replication can experience more lag time due to longer network channels between AWS regions. Check the replication lag to see if data is still being transferred.

2. Binary logging: Ensure that binary logging is enabled on the source RDS instance. For cross-region replication to work properly, binary logging must be enabled on the master database.

3. Network connectivity: Since you're using VPC peering across different AWS accounts, verify that the VPC peering connection is properly configured and that the necessary routes are set up in both VPCs' route tables.

4. Security groups and network ACLs: Check that the security groups and network ACLs allow the necessary traffic between the master and replica instances.

5. RDS instance type and storage: Ensure that the replica has sufficient resources (compute and storage) to handle the replication process. If the replica has significantly less resources than the master, it might struggle to keep up with data transfer.

6. Replication errors: Check the RDS logs for any replication errors that might be preventing data transfer.

7. Initial snapshot transfer: During the creation of a cross-region read replica, Amazon RDS should take a snapshot of the source cluster and transfer it to the replica's region. Verify if this process completed successfully.

If you've checked these points and the issue persists, you may need to contact AWS support for further assistance. They can help investigate why the initial data transfer didn't occur as expected and guide you on how to resolve the issue.
Sources
Question about creating a read replicated database instance | AWS re:Post
Replicating Amazon Aurora MySQL DB clusters across AWS Regions - Amazon Aurora

Just to note, RDS replication does not use your VPC peering.

RDS replicates across an AWS backbone and therefore VPC Peering, Security groups ETC do not come into play here.

There are objects that will not be replicated from the Primary to the Read Replica. I.e., some objects from master will not be on the read replica. I think master may be created from fresh in this instance.

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/SQLServer.ReadReplicas.ObjectSynchronization.html