nginx proxy reveals /wp-admin and /wp-login backend url and port number

I am currently trying to use Nginx as a reverse proxy and as my back-end file server in place of Apache. The goal is to only use the worpress-api as a backend service for my Express.js website.

When passing requests using /admin and /login the backend server path and port number is revealed.

Expected behavior:

http://domain_name/admin ===> http://domain_name/admin http://domain_name/login ===> http://domain_name/login Actual Behavior:

http://domain_name/admin ===> http://domain_name:8080/wp-admin/ http://domain_name/login ===> http://domain_name:8080/wp-login/

Here is my Nginx default config:

upstream file_server {
    server 127.0.0.1:8080;
}

# file-server block
server {
    listen 8080;
    server_name localhost;
    root /var/www/headless-cms/admin;
    index index.php index.html index.htm index.nginx-debian.html;

    location / {
        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;
    }
}

# proxy server block
server {
    listen 80 default_server;
    listen [::]:80 default_server;

    server_name domain_name;

    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

    location / {
       # settings for express.js server
       # proxy_pass http://localhost:5000/;
       try_files $uri $uri/ =404;
    }

    location /api {
        proxy_pass http://file_server/wp-json/;
    }

    location /login {
        proxy_pass http://file_server/wp-login.php;
    }

    location /admin {
        proxy_pass http://file_server/wp-admin/;
    }

}