Debian 10 Digital Ocean VPS. Webmin 1.982 and Usermin 1.832.
I have Virtualmin installed to handle the email components only. The corresponding webservers are handled by another VPS.
I have virtual servers of domain1.com and domain2.com. I have LetsEncrypt certs for mail.domain1.com and mail.domain2.com.
mail.domain1.com and mail.domain2.com both resolve to the email VPS, and sending/receiving emails work fine.
When I add set up domain2.com on a mail client (both Spark and Thunderbird), it finds the certificate for domain1.com.
Domain1.com is alphabetically before domain2.com, which is the only reason why I can imagine it’s finding that one.