Freron MailMate

I have a number of folders of archived mail which, due to how the mail was collected, contains many duplicate messages. In the past, the Edit ▹ Select Duplicates function worked, and I remember I was able to use that to deduplicate folders in past years. At some point in the last couple of years, this MailMate function seems to have stopped working, and now when I try it, no messages are selected. I've tried repeatedly, at different times, also with different recent versions/updates of MailMate, so it's not a recent change in behavior. I kept thinking I'm doing something wrong, but at this point, I think it's time to ask for help.

The procedure I'm following:
1. Click on a folder in the sidebar
2. Select Edit ▹ Select Duplicates
3. Look for any selected messages

The folders I have are filled with duplicates (as determined by the Message-ID header, the subject, the sender, and the date), so it would be immediately obvious if duplicates were selected. There is no ambiguity, and I'm not simply overlooking selected duplicates. I'm staring right at them.

Am I doing it wrong? Or has the function stopped working? Or is there a combination of things that must be true (or not true) to let it work, and I'm overlooking it?

Any help would be most welcome.

I'm using MailMate version 1.14 (5964) on macOS Ventura 13.3.1 on an M2 MacBook Pro.

Best regards,
Mike