Question:
User could see the shared Microsoft365 mailbox in the Outlook365 Desktop client but it read as empty.
Answer:
Microsoft365 adds shared mailboxes to Outlook desktop clients automatically. It can take up to 5 min for it to show up after initial creation. Though this problem occurred within a few days.
Another tech (onsite employee) had added the shared mailbox manually, resulting in problems when the automatic mailbox was added.
Removal of the manually added mailbox, then restarting Outlook solved the issue.
Software specific report (3rd party programs like Quickbooks, Sage, Office365, etc)