Monterey OSX Mail Search does not work

I know this has been written up elsewhere, but I wanted to share my experience in case the pattern suggests a fix to anyone.

After upgrading to Monterey, my mail search does not work. This is what happens and stays this way:

I have tried the following:

1. Telephone calls with Apple support and Senior Apple support (who both did screen shares) - they could not help and said would refer the problem to engineering.
2. I rebuilt my mailboxes by deleting all envelope**** files in ~/Library/Mail/V9/MailData. The search above "oura" worked the first time post rebuild, but has stopped working again. Strange behavior indeed!

Any suggestions would be very welcome - many thanks!