Next weird thing for me... If I restore everything, and let only DNS record for subdomain, it shows me the content of the main domain. I don't have any default server, it's OK?

Can you share the two server blocks you have configured?

You can also rule out a DNS issue by setting a temporary hosts file entry for your new subdomain. This will allow you to make sure that nginx is handling the name correctly. If things work properly with the hosts file entry I would double-check your DNS settings.

Main domain

server {
        listen 80;
        listen [::]:80;
        server_name maindomain.com www.maindomain.com;
        return 301 https://$server_name$request_uri;
}

server {
        client_max_body_size 5M;

        listen 443 ssl;
        listen [::]:443;
        include snippets/ssl-maindomain.com.conf;
        include snippets/ssl-params.conf;

        root /var/www/maindomain.com/public;

        index index.html index.htm index.nginx-debian.html index.php;

        server_name maindomain.com www.maindomain.com;

        location / {
                #try_files $uri $uri/ =404;
                try_files $uri $uri/ /index.php$is_args$args;
        }

        location ~ \.php$ {
                include snippets/fastcgi-php.conf;
                fastcgi_pass unix:/run/php/php7.0-fpm.sock;
        }

        location ~ /\.ht {
                deny all;
        }

        location ~ /.well-known {
                allow all;
        }

        location = /favicon.ico { log_not_found off; access_log off; }
        location = /robots.txt { log_not_found off; access_log off; allow all; }
        location ~* \.(css|gif|ico|jpeg|jpg|js|png)$ {
                expires max;
                log_not_found off;
        }
}

Subdomain

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    root /var/www/html;

    # Add index.php to the list if you are using PHP
    index index.html index.htm index.nginx-debian.html;

    server_name sub.maindomain.com;

    location / {
        # First attempt to serve request as file, then
        # as directory, then fall back to displaying a 404.
        try_files $uri $uri/ =404;
    }

    # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
    #
    location ~ \.php$ {
        include snippets/fastcgi-php.conf;

    #   # With php7.0-cgi alone:
    #   fastcgi_pass 127.0.0.1:9000;

        # With php7.0-fpm:
        fastcgi_pass unix:/run/php/php7.0-fpm.sock;
    }

    # deny access to .htaccess files, if Apache's document root
    # concurs with nginx's one
    #
    location ~ /\.ht {
        deny all;
    }

    location ~ /.well-known {
                allow all;
        }
}