CloudFormation - Aurora Fast Cloning

Hi,

I'd like to create a provisioned clone of my serverless Aurora MySql database using CloudFormation. The docs state that this should be possible, and it is when using the AWS CLI. However, when I try to accomplish this using CloudFormation, I always end up with a serverless clone instead of a provisioned clone. Here's my cluster definition:

"Type": "AWS::RDS::DBCluster",
"Properties": {
    "SourceDBClusterIdentifier": {
        "Ref": "SourceDbClusterIdentifier"
    },
    "RestoreType": "copy-on-write",
    "EngineMode": "provisioned",
    "Engine": "aurora-mysql",
    "UseLatestRestorableTime": true,
    "DBClusterIdentifier": "WhyAmIServerless"
}

There's also a AWS::RDS::DBInstance attached to this cluster, but the stack errors out telling me I can't add an instance to a serverless cluster. Fair enough, but why is this cluster serverless in the first place? It seems like the EngineMode property is being ignored. Does anyone know if creating a provisioned clone from a serverless cluster is possible in CloudFormation? Thanks.