Cross-Tenant Mailbox Migration - HTTP 401 Unauthenticated during Test-MigrationServerAvailability

We are performing a Cross-Tenant Exchange Online mailbox migration between two Microsoft 365 tenants.

The migration endpoint is configured with ApplicationId authentication and validates successfully.

When running:

Test-MigrationServerAvailability -Endpoint "MexicoToPortugal" -TestMailbox "<user>"

the test consistently fails with:

StatusCode="Unauthenticated"

HTTP Status Code: 401

We reproduced the issue with two different users.

We have already validated:

- MailUser configuration

- ExchangeGuid

- LegacyExchangeDN (X500)

- ExternalEmailAddress

- Accepted Domains

- Organization Relationships

- Migration Endpoint

- Enterprise Application

- Mailbox.Migration permission

- Admin Consent

- Cross-Tenant Migration licensing

The same HTTP 401 error occurs for multiple mailboxes after all configuration issues are corrected.

Has anyone experienced a similar Cross-Tenant Mailbox Migration scenario where Test-MigrationServerAvailability returns HTTP 401 Unauthenticated even though the Enterprise Application, Mailbox.Migration permission and Admin Consent are correctly configured?