NEW Data Point?
I had experienced the problem first shortly after upgrading to Big Sur. Then - thank Heaven - all my searches just began working consistently and continued working until today.
Today - NOTE: I've since upgraded to Monterey v12.0.1 (21A559) and many months have passed since Big Sur and the Monterey update - I experienced the most disruptive behaviour when searching in MAIL: it would find only those messages that had been sent to me today. All the historical messages were in my INBOXes but they wouldn't be found regardless if I selected ALL INBOXES or ALL MAILBOXES (well, mostly my EXCHANGE and iCloud messages were missed. Other messaging service INBOXes worked a bit.).
I tried various workarounds including restarts without success.
Later this afternoon I noticed that I had 10 pending application updates. So, since mail was being frustrating and I was tiring of manually searching my INBOXes, I did the application updates and, because it's a habit of mine, I then did a complete restart of my macOS machine.
When it came back up MAIL searches were working.... ALL Mail searches were now finding anything I wanted as it had done before the problem expressed itself today.
I find it difficult to believe that pending application updates would break MAIL Search but...... well, I thought I'd mention it.