In this scenario, an on-premise user was created and synced with Office 365. The Office 365 user has an Exchange Online mailbox, but this mailbox is not visible in the on-premise Exchange Admin Center.
Adding users and mailboxes changes slightly when working in hybrid Exchange environments.While adding a user in ADUC and licensing the user in Office 365 is possible, the user's Exchange attributes will never be synced with the on-premise environment.
Converting a user mailbox to a shared mailbox in a hybrid Exchange environment can be frustrating. Fortunately, there is a PowerShell command available that makes the entire process much simpler.