Invalid Proxy addresses

Previously when migrating mailboxes to Exchange Online in a hybrid scenario, if the mailbox had SMTP addresses that were not registered in the Office 365 tenant the migration would fail and these addresses would need to be removed from the on-prem user account. Now it seems that the migration succeeds with the EXO mailboxes not having the invalid SMTP addresses configured. Is anyone able to point to anything published by Microsoft on this apparent change in behaviour at all?