I see that you have a case open with support so they should be able to help you further based on what they see in the logs. Without those logs I would be guessing here
Discovering if the mailbox is moved to the cloud would make sense but is harder to achieve than it looks. Unless you have a connection between AD and AAD it is difficult to figure that out (even if the mailbox remains the same, the GUID and such are different).
Please keep us informed about the status of the support call