In a Microsoft Exchange Server 2019 or Exchange Server 2016 environment, assume that you manually add a shared mailbox as an additional account for your primary mailbox in Outlook 2019 or Outlook 2016. You may only get the search results from the primary mailbox and no search results are returned from the shared mailbox.
To fix this issue, updates for both Exchange Server and Outlook are needed.
For Exchange Server, install one of the following updates:
For Outlook 2019, install the latest update.
For Outlook 2016, upgrade to Outlook 2019 and install the latest update.
Learn about the terminology that Microsoft uses to describe software updates.