When you search in Outlook using All Mailboxes or All Outlook Items scopes, limited or no results are returned for the Archive Mailbox. 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.
