Try google - it shows related issues:
https://support.office.com/en-us/article/Fixes-or-workarounds-for-recent-issues-in-Outlook-for-Mac-54afa5e3-db38-422a-9d94-3b55330ded8e
Look under the 'Automated troubleshooting tools'
Outlook Search Repair
This tool repairs search issues in Outlook for Mac. For example, when search returns no results for items that do exist in your mailbox, or you get older data when you have newer items.