I had the same issue after upgraded to Monterey 12.1. Outlook search function was working fine but stopped working after upgraded the OS, even the Recents tab on finder's sidebar, search Applications also stopped working. I tried to reindex, reinstall MacOS, but issue was still persist. So I had to fresh reinstall MacOS (Erase all contents and settings then setup all over again) to make it works again. I am using Macbook pro 14 M1 Pro.
leehai013
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You are right, it can search for few emails while indexing then stopped again after index completed. @_@.