A couple of times, I've had new subdomains display the web site for a different subdomain.
- At first, my "mail" subdomain was showing my "www" subdomain's content.... well, OK, I didn't have anything defined for my "mail" subdomain.
- So I added forwarding to send my "mail" subdomain to the 2079 port for webmail. Worked great.
- Now I needed to do drupal testing, so I created a "drupal" subdomain. But now my "mail" subdomain is showing the drupal home page.
I can see the DNS records for all the subdomains -- "A" type records to the same IP address. (Some were CNAME, but I tried changing to "A" in case that matters.)
I'm pretty sure this is a webserver configuration issue, but cPanel should be taking care of that. I'm surprised that I'm not finding reports of similar problems in any of the forums (so I'm probably using different terminology).