can't migrate a synced user

Hi, we have a hybrid infrastructure with Exchange 2013. We had a linked mailbox, which I converted to an user mailbox. Then I successfully synchronized this user to O365, but I can't migrate it: the wizard doesn't see this mailbox at all.

I created another test mailbox, synced it and I see it in the wizard. Why I don't see all synced mailbox in the migration wizard?

thank you in advance!