Excluding subdomains with nginx and 301 redirect to https?

I recently moved a website from Dreamhost to an Ubuntu LEMP droplet. Much, much faster. This also allowed us to enable HTTPS using our preexisting wildcard certficate. I’ve setup a redirect to move all incoming traffic on http://[www.]domain.tld:80 to https://[www.]domain.tld:443 like so:

server {
       listen         80;
       server_name    domain.tld www.domain.tld;
       return         301 https://domain.tld$request_uri;
}

This works really well. Too well! I still have a few subdomains hosted at Dreamhost, which are HTTP only. Now any browser that first visits domain.tld always prefixes https:// instead of http://. I understand this working on the explicit www.domain.tld and domain.tld, but why is this 301 redirect being applied by browsers to all subdomains? Is there anyway to do this differently so that unspecified subdomains are not included in the 301?