AWS RDS instances have copy tags to snapshots feature disabled which may not allow you to add metadata and apply access policies to your AWS RDS resources.
In AWS Console -
In Terraform -
References:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_CommonTasks.BackupRestore.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/rds_cluster#copy_tags_to_snapshot