Had the same issue for over a year. Tried everything without success.
Saw this on another forum, tried it and now works perfectly!
Outlook Support:
I will be glad to assist you. With regards to the search issue, let's run a couple of commands on Terminal to manually initiate indexing and check if that helps:
Open Finder and go to Applications - Utilities - Terminal and type the following commands (Press Enter after typing each command).
sudo mdutil -a -i off
sudo mdutil -a -i on
Note: When prompted for a password, please type the Mac user password. This would be invisible. Please type the same and hit enter.
Once done, restart the mac computer and wait for about an hour and check for the behavior.
Please try and let us know the Outcome. Thank you.