Webmin's Let's encrypt SSL certificate is not automatically renewed

Operating system Ubuntu Linux 18.04.5
Webmin version 1.973
Virtualmin version 6.16

Under Webmin > Webmin Configuration > SSL Encryption > Let’s Encrypt, I’ve configured automatic renewal as below:

Hostnames for certificate: my.domain.com
Website root directory for validation file: Other directory: /var/www/html
Copy new key and certificate to Webmin? Yes
Let's Encrypt server: Real
Months between automatic renewal: 1

When I request certificate manually, it gets renewed successfully.
But certificate never gets renewed automatically every month as expected.

I have this issue with Webmin’s certificate itself only.
My virtual servers’ certificates are successfully renewed automatically (though Virtualmin).

EDIT:
I had the same problem with virtual servers’ certificates before changes introduced in Virtualmin fixed it.
See Webmin : Version 6.16 (28th March 2021) “Let’s Encrypt auto-renewal has been simplified and is on by default.”
Unfortunately the same change was not applied to Webmin’s auto-renewal :frowning: