Simply shutting Outlook and then reindexing (and waiting until that finished before reopening Outlook), worked for me. Bizarre, I know, and hard to imagine I hadn't had that configuration before, out of the dozens of times I've reindexed and attempted the other tactics here, all to no avail. And while, inexplicably, it seems Mac OS (Big Sur) provides no ready way to track reindex status, I did stumble upon a trick. After you've set off the reindex, click on Spotlight icon and, without hitting return, type "reindex" into the Spotlight Search window -- for me, this prompted a status bar to display.