Well, I found this article, which seemed quite promising …
Calendar Search not Working on Mac, Fix
It didn't solve my Calendar problem, but maybe it'll solve someone else's.
FYI, Events & Reminders are showing up in my Spotlight searches but not in my Calendar searches.
Using the search term 'renew,' here's an example from Spotlight …
I just noticed that all three of these events appear to take place on July 17 (what year is unclear), but when I repeated the Spotlight search and clicked on the individual events, none of them was actually on July 17 and all were in different years.
Using the same search term in Calendar, here's what happens …
And the "fix" in Safe Mode (suggested in the article at the top) had no effect.
So, I kept looking and came across this in the iCloud User Guide:
Restore your calendars and events on iCloud.com
I restored my calendar to the last iteration before the migration.
That didn't solve my problem; search is still not working in Calendar.
I hope it didn't create any new problems. Keeping my fingers crossed! 🤞
As stated above, all insights are welcome.