We have created around 20,000 MailUser objects, but have only migrated 9. We have 50 scheduled for next week and noticed these 2 “missing”
Azure AD changes were taking hours to replicate to ExO
Based your description, although AAD Connect sync is unstable, it still works. I guess this phenomenon caused by this account broken or there exist sync issue between your AAD and Exchange online：
About the account broken issue, you could try to delete this MailUser from Exchange on-premise, then try to recreate a new one, check whether this new on could show in Exchange online after syncing.
About the sync issue, you could open a service request from Microsoft 365 admin center to let Microsoft to help you check from the backend.
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