In a Microsoft Exchange Server 2016 hybrid environment, you could still run the Enable-Mailbox cmdlet against a remote mailbox that's already migrated to Microsoft Office 365, and provision a mailbox at the on-premise Exchange server.
This issue occurs because the validation is disabled.
To fix this issue, install Cumulative Update 9 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.