Searching Shared Mailboxes on MacOS is not showing same result as separate accounts

When I have 4 normal mail accounts in Outlook on MacOS, I can search through all Inboxes ('Alle postvakken') and will find all related mails over all mailboxes.

When I change those mailboxes to Share Mailboxes and add them to Outlook in that way, the searching is no longer working as before. From that moment on I only find some results in the currently selected mailbox.

How can we get Outlook (on MacOS) to search through all delegated mailboxes that are vissible in Outlook?