How to set a rule in nginx only for a subdomain?

Question

Nginx

Problem: https://domain.com redirects to http://domain.com/attempt/login/to/seafile
Given:
root domain domain.com
subdomain https://sub.domain.com/ - seafile server , their manual config via nginx:

server {
      listen       80;
      server_name  sub.domain.com; 
      rewrite ^ https://$http_host$request_uri? permanent;
  }

  server {
      listen 443;
      ssl on;
      ssl_certificate /etc/ssl/cacert.pem;
      ssl_certificate_key /etc/ssl/privkey.pem;
      server_name sub.domain.com;    
      # длинный конфиг #
      fastcgi_param   HTTPS               on;
      fastcgi_param   HTTP_SCHEME         https;
  }

1. Why does it work not only with sub.domain.com, but also tries to do something with the root domain?
2. How to fix it?
3. How to leave https://domain.com on request without redirects to seafile?