Express req.subdomains empty

I have this in the Windows hosts file:

127.0.0.1       demo.app.com

And when I test it at http://demo.app.com:5000/, req.subdomains returns ["demo"].

However, when I deploy it to an Ubuntu server, the req.subdomains returns [].

What gives?

server {
    listen 80;
    server_name *.app.com;
    location / {
        proxy_pass 'http://127.0.0.1:5000';
    }
}