i've done this exercise before, of moving mailboxes to another database. i tried different destination databases but a certain mailbox just won't get moved. the migration job would finish within a few minutes (the mailbox is around 5GB) with zero work done. it just says successful (zero synced, zero finalised, zero failed).
nothing in event viewer points or related to this strange behaviour. i thought of maybe because of the backup but it's already done and still i can't move the mailbox.