Starting from March 2021 Virtualmin stopped sharing Let's Encrypt wildcard SSL certificate for the new subdomains created as "Top-level server".
So, when the new subdomain is being created as top-level server it requests individual SSL certificate from Let's Encrypt now and doesn't share already existent wildcard Let's Encrypt SSL certificate for such subdomain anymore.
But wildcard SSL certificate sharing works without problem if subdomain is created as "Sub-server" within top-level server,
I can confirm this issue on Debian 9 and Debian 10 with the newest Webmin/Vitualmin packages versions.
Option "Share SSL certificates between domains where possible?" is enabled ("Yes") in Virtualmin Configuration.