Hybrid Office 365 setup. User account is created on-premise, AD is synced, and then the user is licenced as an Office 365 user. Exchange won’t create the Remote User forwarder, so the user will only receive email from Office 365 (and external if the MX record has been switched to Office 365).
You can join the account up with the command:
Enable-RemoteMailbox Username -RemoteRoutingAddress “firstname.lastname@example.org”
That will create the Remote Mailbox on Exchange with the forwarder email address.