Question
Why does whm.domain.tld not have an SSL certificate even though cpanel.domain.tld is secured?
Answer
These subdomains (cpanel., webdisk., webmail., whm., etc) for domain.tld are set up using the Service Subdomain Feature, however, the WHM subdomain is only enabled for reseller users.
Though AutoSSL is capable of issuing an SSL cert for whm.domain.tld, the Service Subdomain Feature does not add the domain to the user's configuration unless they are a reseller.
This is an intended outcome when using the Service Subdomain Feature.
For more information, please see our documentation:
Access interfaces | Service and Proxy Subdomains (cPanel Docs)