Export feature in Mail.app literally does nothing?

I used to use the export feature in Mail.app fairly frequently to back up my GMail and some other email to a local disk.

Now the export feature seems to do almost literally nothing.

No matter what account, folder, sub-folder, or "mailbox" I choose, when I export it, I end up with 3 files: Info.plist and table_of_contents, both of which are trivially small (maybe 30 to 200 bytes) and an mbox file, which should contain my emails, but it's zero bytes.

Is anyone else seeing this problem?