I manage a hybrid Exchange environment with 2 x Exchange 2016 servers (CU19) on-premise. I ran the HCW some time ago, and had to do lots of things manually that it couldn't do (such as create connectors), but for the most part it seems to work well.
I have created some shared mailboxes on-premise and granted access to some users. They appeared in Outlook and all was well. I have now migrated these mailboxes to Exchange Online and while existing users can still access them new users can't. I've delegated permission through the usual way but they can never see the mailbox. Also, I've revoked access for some users but they can still access them.
What am I missing, what's not working or not setup right to cause this to happen?