Domain Access and cPanel v102.0 (build 28)

Ok, problem is solved, for me, but it is not a general solution. (I'll describe what worked after this brief discussion -- I offer all of this for others who may run into the same issue)

A general solution would require offering an optional significant implementation change to Domain Access, something I doubt @agentrickard would consider viable. It would require that each subdomain be mapped to its own drupalroot/subdomain folder, instead of just drupalroot.

The subdomain creation tool in my current version of cpanel, found at https://my.bluehost.com/hosting/app/#/domains/subdomains, does not allow specifying public_html to be the home directory of a subdomain. It forces a folder inside public_html to be the home of subdomain.

I found that instead of using the subdomain creation tool, I could simply add an "A" record in DNS settings, pointing to the server's IP address, and that would allow a new DA domain to be created and work.

However, my wildcard certificate was not getting notified that subdomain.example.com exists, so users would get a SSL_ERROR_BAD_CERT_DOMAIN error trying to reach https://subdomain.example.com

I found an SSL/TLS Status applet in cPanel that, through an AutoSSL tool, allows managing all the sites that share my wildcard certificate. All sites created through the subdomain creation tool are listed, but sites created by just adding a DNS settings "A" record do not. If there were a way to tell AutoSSL about my new "A" record, that might also solve the problem. I could not find a way to do this.

I contacted Bluehost a year ago about this and they suggested the "A" record approach. I contacted them again today and this time they let me know the old cpanel subdomain creation tool was still available, at https://my.bluehost.com/cgi/dm/subdomain. With this old tool available, I deleted the DNS "A" record and created the subdomain having its home directory be drupalroot and now the subdomain works properly in Domain Access, including via https