Hi jaumont,
I would like to confirm if you set up Windows Azure Active Directory Sync tool. If so, by default, the attribute SamAccountName will be synced to Office 365. However, we can filter the attribute via miisclient. We can locate at C:\Program Files\Windows Azure Active Directory Sync\SYNCBUS\Synchronization Service\UIShell\miisclient.exe. Then uncheck the attribute as below screenshot:
Thanks,
Johnny Zhang