In my case (Ventura/Outlook 2016) the following fix did solve the issue:
Step 1)
sudo mdutil -a -i off
sudo mdutil -a -i on
Step 2) Re-Index your HDD
Open System settings and select Siri and Spotlight.
Scroll down and click the Spotlight Privacy.
Click on Go - Computer.
Drag the Macintosh HD to the Privacy tab and wait for some time.
From the Privacy window, select the HD and then click the minus (-) button near the bottom left of the window.
Step 3) Enable && Disable "New Outlook" Feature
Step 4) Have fun with the search feature
Seems to be a new "New Outlook" feature :)
By the way, I tried a lot fixes including
mdimport -r "/Applications/Microsoft Outlook.app/Contents/Library/Spotlight/Microsoft Outlook Spotlight Importer.mdimporter" -d1 "/Users/my_user/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles"
no effect :)
deepblue666
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Paderborn