Most answer here are rather heavy-handed. The link provided by OP has apparently been updated and now suggests to reset the Spotlight index for the Outlook folder. This is a much less intrusive solution, takes about 20 minutes of rebuild time, and often works. Therefore, I would recommend to everybody to first try this method.
Quote from the Microsoft support article [with additions by myself]:
Choose Apple menu, then System Preferences, and then Spotlight.
Select the Privacy tab.
a. On the Go menu, select Go to Folder...
b. Copy and paste the following location into the "Go to the folder:" dialog box and select Go:
~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/
Drag the "Main Profile" folder to the Privacy tab. Once added, remove the folder [from the Spotlight privacy tab], and Spotlight will re-index the folder.
You must perform this step for any additional profile folders you have.
See the following article from Apple to learn more: How to rebuild the Spotlight index on your Mac.
After that macOS will rebuild the index for that folder, which will take a while (up to an hour approximately) depending on your Mac's speed and the size of your Outlook database.
If you suspect that there is a more severe issue with Spotlight, have a look at this answer on how to reset Spotlight completely. Again this is a rather drastic measure and may take a few hours to rebuild.