AWS Backup - Copying backups across accounts in separate AWS Organizations - Is it possible?

I’m trying to validate a cross-organization backup copy scenario and would appreciate clarification.

Scenario

• Account A1 in Org A (Production Org)
• Account B1 in Org B (Recovery Org)

In Account B1, I created a Logically Air-Gapped Vault (LAGV) using a custom KMS key. In the KMS key policy, I allowed the AWS Backup service principal. Using AWS RAM in Account B1, I shared the LAGV with Account A1 (in Org A). After sharing, I see the vault under “Vaults shared through RAM” in A1. In the LAGV vault policy, I allowed Account A1 to copy backups into the vault. To eliminate permission-related issues during testing, I also temporarily allowed "Principal": "*".

Vault policy (for reference):

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowProdCopyIntoLAGV-1",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::<A1-account-id>:root"
            },
            "Action": "backup:CopyIntoBackupVault",
            "Resource": "*"
        },
        {
            "Sid": "AllowProdCopyIntoLAGV-2-dangerous-only-for-testing",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "backup:CopyIntoBackupVault",
            "Resource": "*"
        }
    ]
}

In the Settings section of AWS Backup (in both organizations), I enabled:

• Cross-account monitoring
• Cross-account backup
• Multi-party approval integration

In Account A1, I created a Backup Plan that:

• Backs up an S3 bucket (all resources are in us-east-1)
• Copies to a local standard vault
• Includes a copy action to another account’s vault (the shared LAGV ARN from B1) - this is done under the Copy section of the Backup Plan
• The service role used is AWSBackupDefaultServiceRole, which has - AWSBackupServiceRolePolicyForBackup and
• AWSBackupServiceRolePolicyForRestores

Result The first step in the backup plan - copying to the local vault in A1 succeeds. However, the second step -copy to the LAGV in B- fails with the error: “Access denied – Copy job failed. Both source and destination account must be a member of the same organization.” I have tested with a custom KMS key and later with an AWS-managed KMS key. The same error occurs in all cases.

Question Is cross-organization copy into a backup vault (including LAGV) not supported at all? I’ve heard AWS representatives suggest using AWS Backup to copy backups into a separate recovery organization for resiliency. Am I missing something here? I also found a similar question asked here a year ago: https://repost.aws/questions/QURX0xbdOPQv6_YdX7sVUGcg/how-to-copy-aws-backup-cross-aws-organization The answer referenced the AWS documentation, which I have gone through but does not provide an answers to my question