Using Classic Outlook on Windows for personal email, calendar, and contact management
I had same problem after upgrading to Windows 11. Tried to resolve the issue with help of Microsoft Support agents, but in vain. Outlook would keep restarting indexing on its own, and emails that I used to find earlier were nowhere to be found in search.
Found a fix for this today.
Very easy method to resolve the issue. You can fix Outlook search not returning any results by setting a registry key that disables Windows Desktop Search and tells Outlook to use its built-in search. Details given in above link.
I have implemented it, and works wonders.
Hope this helps.