In a mailbox level backup, you are able to see the list of mailboxes in the local Monitor, but some of the mailboxes that are on the server are missing from that list.
This issue is caused by incorrect or damaged MTA values, often caused by a migration from an earlier version of Exchange.
The following article from Microsoft describes how to address the issue and repair the MTA values: Error message in Exchange Server: The Delegate page is not available. Unable to access Outlook folder
All mailboxes appear on the mailbox configuration list.