Hi @dennis Chang ,
Thanks for your reply and good sharing! IAgree with you, if we are searching in the Archive Mailbox, we need to select Current Folder option as the search scope.
However, due to a recent update in forum policy, the question author now is not able to accept their own answers. I have written a brief summary of this issue. Please feel free to accept it as the answer to the question to help other community members. Thanks in advance.
Issue Symptom:
When searching in Archive Mailbox, Outlook search time too long and returns incomplete results
Solution:
You can search for items that were moved to an auto-expanded archive mailbox only by searching the folder itself. Select the archive folder in the folder list to select the Current Folder option as the search scope. If a folder in an auto-expanded storage area contains subfolders, you have to search each subfolder separately. For more information, see Overview of unlimited archiving in Microsoft 365.