MariaDB snapshot export to S3 succeeds but is missing all data -- DATABASE_IS_EMPTY

I've just tested RDS MariaDB snapshot export to S3. The snapshot is good, a test restore re-creates the database correctly. However, exporting that snapshot to S3 appears to succeed but no data is exported! The job summary and the metadata stored in the bucket both say the test DB was skipped, and the metadata claims the DB is empty (it isn't).

I'm working with a minimal test case and following the AWS Console process as documented by AWS, including export all data (not a partial export). However, the resulting S3 export contains only the JSON meta-data files; no data appears in S3.

Is this a known problem with RDS MariaDB service?

export_tables_info_MyDbName-snapshot-test-2-after-stopping_from_1_to_1.json:

{
    "perTableStatus": [
        {
            "warningMessage": {
                "skippedDatabase": [
                    {
                        "reason": "DATABASE_IS_EMPTY"
                    }
                ]
            },
            "target": "MyDbName"
        }
    ]
}