That can happen when Spotlight crashes while indexing your Mac, or with weird Spotlight mdimporter conflicts. Sometimes, unfortunately, that’s not enough. This will trigger a proper reindexing of the folder. Most of the time, you can correct the issue by simply adding the ~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile folder to the Spotlight Exclude list in the Spotlight System Preferences, then a few minutes later, remove it from the list. Outlook uses Spotlight for the searches and most of these issues are related to Spotlight indexing issues. A number of people are frequently reporting searches or filtering issues in Outlook 2016.