The new macOS Monterrey 12.1 (21C52) broke the Spotlight search functionality in Outlook 16.56 (21121100). In the new Outlook UI, searching works perfectly fine in the new UI and so does Spotlight search via macOS. Spotlight does not work when it’s invoked from the old Outlook client for some reason (I’m not sure what systematic call fails.)
I suggest everyone to open a case with Outlook Support to receive more traction and visibility.