I managed to set up our Exchange hybrid environment (Exchange 2016 on-premise) and it seems to be working OK. I can migrate users from on-prem to cloud, send messages between them, etc.
The only thing I'm not sure about is viewing the mailboxes at both sides. When I open my on-premise ECP I can see all mailboxes, on-premise and cloud. However, in the Exchange Online control panel I can't see the on-premise mailboxes, just the ones that have been migrated.
Is that correct, is that the desired behaviour? Or is something not working and I should see the on-premise mailboxes in Exchange Online?