Ngixn load balancer keep changing original URL to load balanced URL - nginx

Question

I have met an annoying issue for Nginx Load Balancer, please see following configuration:
http {
server {
listen 3333;
server_name localhost;
location / {
proxy_pass http://node;
proxy_redirect off;
}
}
server {
listen 7777;
server_name localhost;
location / {
proxy_pass http://auth;
proxy_redirect off;
}
}
upstream node {
server localhost:3000;
server localhost:3001;
}
upstream auth {
server localhost:8079;
server localhost:8080;
}
}
So what I want is to provide two load balancers, one is to send port 3333 to internal port 3000,3001, and second one is to send request to 7777 to internal 8079 and 8000.
when I test this setting, I noticed all the request to http://localhost:3333 is working great, and URL in the address bar is always this one, but when I visit http://localhsot:7777, I noticed all the requests are redirected to internal urls, http://localhost:8080 or http://localhost:8079.
I don't know why there are two different effects for load balancing, I just want to have all the visitors to see only http://localhost:3333 or http://localhost:7777, they should never see internal port 8080 or 8079.
But why node server for port 3000 and 3001 are working fine, while java server for port 8080 and 8079 is not doing url rewrite, but only doing redirect?
If you see the configuration, they are exactly the same.
Thanks.