Sorry, I just realised the From being generated is <webmin@foo>, not root@…
Here are a couple of logs showing before and after the upgrade. And I should note, I was a couple of versions down, so this may have been a problem in a version prior to 1.940.
/var/log/mail.log.1.gz:Jan 9 00:09:55 foo postfix/cleanup[29843]: 28093203B0: warning: header Subject: Completed backup of Virtualmin on foo.domain.tld to /var/backups/remote/vsites/foo/2020-01-09 on SSH server bkp.domain.tld from local; from=<webmin@foo.domain.tld> to=<admin@domain.tld>
/var/log/mail.log.1.gz:Jan 10 00:03:49 foo postfix/cleanup[20471]: 395A2201AD: warning: header Subject: Completed backup of Virtualmin on foo. to /var/backups/remote/vsites/foo/2020-01-10 on SSH server bkp.domain.tld from local; from=<webmin@foo> to=<admin@domain.tld>