Apple Mail does not find messages with a given subject
Mac Pro (Late 2013) running Big Sur (11.3.1) I am trying to find all emails whose subject contains SS1809 . I type exactly that in the search bar and select "All Mailboxes" on the left. It returns nothing. So I find the folder in which I expect mails with such a subject and sort by subject and see about a dozen emails beginning with precisely that string. Then I start typing and after typing the single character S it finds 0 results. If I select that mailbox before searching, then start typing it finds a handful of messages with SS in their subjects but ONLY in folders "Deleted Messages" and "Recovered Messages". I have tried restarting, starting Mail with the shift key held down (as was recommended in some old conversations, turning Spotlight's searching off and on from the command line in Terminal.
Totally broken. Richard
Mac Pro, macOS 11.3