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deleting and recreating the stack gave me the following error message that helped solve the problem:
``` DbSubnetGroupName should not be specified for read replicas that are created in the same region as the master```

I suspect the original error was perhaps because a  a non-zero `backup_retention_period` property was added to the source DB after the DB was initially deployed but the `apply_immediately` property was not set.
So the adding of a backup retention period to the source DB was scheduled for your next maintenance window. And, so the read replica was failing due to the source not yet having backups set.
Redeploying the stack avoided this since the source DB was deployed with the backups set and so then you saw the region error.

Thanks <@U01BSQWRZFA>, that’s great insight!