Outlook version? 16.70 current (Feb 14)? 16.71 beta preview channel available. In the past, a re-install has helped some. Packages here:
https://learn.microsoft.com/en-us/officeupdates/update-history-office-for-mac
There are threads here that have tips - from simple to series of terminal commands - for forcing Outlook to reindex via Spotlight. Or even Show Package Contents, Library and launching the Outlook importer from there.