Hi, is the shared mailbox configured to cache downloaded folders?
If so, uncheck that option in Outlook and restart Outlook and search again:
https://support.microsoft.com/en-us/topic/performance-and-synchronization-problems-when-you-work-with-folders-in-a-secondary-mailbox-in-outlook-d45e5881-3d32-ca00-6338-5962cfc41ea8