You can also try switching off the spotlight and turning it on again to fix the issue. Restarting the device will solve the issue.
If Spotlight crashes on macOS it may result in an error in search results.
So Spotlight is forced to re-index the Outlook folder. The above process adds Outlook data folder to the exclusion list for searching, and hence removes the index from the Spotlight.
Lets see how to fix Outlook search issues in Mac OS Latest versions.