It's possible the source mailbox was previously migrated from another source environment or from on on-prem environment and has incorrect attribute values.
If using ODM-AD Directory Sync and synching Exchange attributes up to the AAD tenant, the ODM migration will interpret these mailboxes as an on-premise Exchange mailbox for the user in scope. ODM will not create the target tenant Exchange mailbox as a result - even though the user was already using an Exchange Online mailbox in the source environment
Check the source user account attributes MsExchMailboxGUID, msExchArchiveGUID and msExchArchiveName and make sure these are for the cloud mailbox, and not from an on-prem mailbox.
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