AWS Backup jobs for EFS file systems are empty

I have a backup plan:

{
    "BackupPlan": {
        "BackupPlanName": "prometheus_dev",
        "Rules": [
            {
                "RuleName": "prometheus_dev",
                "TargetBackupVaultName": "xxxxxxxxxxxx",
                "ScheduleExpression": "cron(15 * ? * * *)",
                "StartWindowMinutes": 60,
                "CompletionWindowMinutes": 180,
                "Lifecycle": {
                    "DeleteAfterDays": 7
                },
                "RuleId": "ffcd7e8c-9b14-4e2b-89f0-d8cbe5b5ae25",
                "CopyActions": [
                    {
                        "Lifecycle": {
                            "DeleteAfterDays": 7
                        },
                        "DestinationBackupVaultArn": "arn:aws:backup:eu-west-1:614797193252:backup-vault:xxxxxxxxxxxx"
                    }
                ],
                "EnableContinuousBackup": false
            }
        ]
    },
    "BackupPlanId": "830e8bb0-e3c6-4a96-8fe9-45a3ab97de40",
    "BackupPlanArn": "arn:aws:backup:eu-central-1:614797193252:backup-plan:830e8bb0-e3c6-4a96-8fe9-45a3ab97de40",
    "VersionId": "YzUxMjhkODgtZTc0MC00NDA1LWEwYzktNmE4NDFhMDE5MTA4",
    "CreationDate": "2023-10-13T13:42:22.048000+02:00",
    "LastExecutionDate": "2023-10-20T11:16:48.451000+02:00"
}

The file system is protected:

{
    "ResourceArn": "arn:aws:elasticfilesystem:eu-central-1:614797193252:file-system/fs-0318e5506f10caf1e",
    "ResourceType": "EFS",
    "LastBackupTime": "2023-10-20T11:22:48.159000+02:00",
    "ResourceName": "prometheus_dev"
}

And backups are taken hourly:

// ...
{
            "AccountId": "614797193252",
            "BackupJobId": "06178992-5DF7-17BA-1E38-C969357B644A",
            "BackupVaultName": "xxxxxxxxxxxx",
            "BackupVaultArn": "arn:aws:backup:eu-central-1:614797193252:backup-vault:xxxxxxxxxxxx",
            "RecoveryPointArn": "arn:aws:backup:eu-central-1:614797193252:recovery-point:58cb7957-5b0c-4038-a579-af78aadbc506",
            "ResourceArn": "arn:aws:elasticfilesystem:eu-central-1:614797193252:file-system/fs-0318e5506f10caf1e",
            "CreationDate": "2023-10-20T08:15:00+02:00",
            "CompletionDate": "2023-10-20T08:25:07.003000+02:00",
            "State": "COMPLETED",
            "PercentDone": "100.0",
            "BackupSizeInBytes": 0,
            "IamRoleArn": "arn:aws:iam::614797193252:role/xxxxxxxxxxxx",
            "CreatedBy": {
                "BackupPlanId": "830e8bb0-e3c6-4a96-8fe9-45a3ab97de40",
                "BackupPlanArn": "arn:aws:backup:eu-central-1:614797193252:backup-plan:830e8bb0-e3c6-4a96-8fe9-45a3ab97de40",
                "BackupPlanVersion": "YzUxMjhkODgtZTc0MC00NDA1LWEwYzktNmE4NDFhMDE5MTA4",
                "BackupRuleId": "ffcd7e8c-9b14-4e2b-89f0-d8cbe5b5ae25"
            },
            "StartBy": "2023-10-20T09:15:00+02:00",
            "ResourceType": "EFS",
            "IsParent": false,
            "ResourceName": "prometheus_dev"
        },
        {
            "AccountId": "614797193252",
            "BackupJobId": "DE456E1F-1A49-4A36-507A-32646DD0AE85",
            "BackupVaultName": "prometheus_eu_central_1_dev",
            "BackupVaultArn": "arn:aws:backup:eu-central-1:614797193252:backup-vault:xxxxxxxxxxxx",
            "RecoveryPointArn": "arn:aws:backup:eu-central-1:614797193252:recovery-point:998837d7-e0c8-4505-9d4f-ff19ca1f69c7",
            "ResourceArn": "arn:aws:elasticfilesystem:eu-central-1:614797193252:file-system/fs-0318e5506f10caf1e",
            "CreationDate": "2023-10-20T07:15:00+02:00",
            "CompletionDate": "2023-10-20T07:25:52.381000+02:00",
            "State": "COMPLETED",
            "PercentDone": "100.0",
            "BackupSizeInBytes": 0,
            "IamRoleArn": "arn:aws:iam::614797193252:role/xxxxxxxxxxxx",
            "CreatedBy": {
                "BackupPlanId": "830e8bb0-e3c6-4a96-8fe9-45a3ab97de40",
                "BackupPlanArn": "arn:aws:backup:eu-central-1:614797193252:backup-plan:830e8bb0-e3c6-4a96-8fe9-45a3ab97de40",
                "BackupPlanVersion": "YzUxMjhkODgtZTc0MC00NDA1LWEwYzktNmE4NDFhMDE5MTA4",
                "BackupRuleId": "ffcd7e8c-9b14-4e2b-89f0-d8cbe5b5ae25"
            },
            "StartBy": "2023-10-20T08:15:00+02:00",
            "ResourceType": "EFS",
            "IsParent": false,
            "ResourceName": "prometheus_dev"
        },
// ...

The problem is that ALL recovery points are empty, even if the file system is not:

{
// ...
            "SizeInBytes": {
                "Value": 67465216,
                "Timestamp": "2023-10-20T10:41:44+02:00",
                "ValueInIA": 0,
                "ValueInStandard": 67465216
            }
// ...
}

I've also tried to start a backup job:

aws backup start-backup-job \
    --backup-vault-name prometheus_eu_central_1_dev \
    --resource-arn arn:aws:elasticfilesystem:eu-central-1:614797193252:file-system/fs-0318e5506f10caf1e \
    --iam-role-arn arn:aws:iam::614797193252:role/xxxxxxxxxxxx

Which produced the following response:

{
    "BackupJobId": "7a010939-11cd-4d6f-bf2e-bbec0fc50452",
    "RecoveryPointArn": "arn:aws:backup:eu-central-1:614797193252:recovery-point:ac338f60-9a4e-4c24-817e-2e0e15a72d03",
    "CreationDate": "2023-10-20T11:22:48.159000+02:00",
    "IsParent": false
}

But again the backup is empty:

{
    "AccountId": "614797193252",
    "BackupJobId": "7a010939-11cd-4d6f-bf2e-bbec0fc50452",
    "BackupVaultName": "prometheus_eu_central_1_dev",
    "BackupVaultArn": "arn:aws:backup:eu-central-1:614797193252:backup-vault:xxxxxxxxxxxx",
    "RecoveryPointArn": "arn:aws:backup:eu-central-1:614797193252:recovery-point:ac338f60-9a4e-4c24-817e-2e0e15a72d03",
    "ResourceArn": "arn:aws:elasticfilesystem:eu-central-1:614797193252:file-system/fs-0318e5506f10caf1e",
    "CreationDate": "2023-10-20T11:22:48.159000+02:00",
    "CompletionDate": "2023-10-20T11:22:55.105000+02:00",
    "State": "COMPLETED",
    "PercentDone": "100.0",
    "BackupSizeInBytes": 0,
    "IamRoleArn": "arn:aws:iam::614797193252:role/xxxxxxxxxxxx",
    "ResourceType": "EFS",
    "BytesTransferred": 0,
    "StartBy": "2023-10-20T19:22:48.159000+02:00",
    "IsParent": false,
    "ResourceName": "prometheus_dev"
}

Why are these backups empty?