Why socket is closed when I had set the keepalive_timeout = 1h in nginx configuration

Question

I wrote a small http server in nodejs to understand the keep-alive behaviour. Here is the source code for same -

const http = require('http'),
  server = http.createServer((req, res) => {
    const socket = req.socket,
      socketId = socket._handle.fd;

    console.info(`Writing response to the socket ${socketId}`);
    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end('okay');
  });

server.on('clientError', (err) => {
  if (err) {
    console.error(err);
  }
});


function getSocketCloseFn (socketId) {
  return function onSocketClose (hadError) {
    if (hadError) {
      console.error(`Found error in the socket ${socketId}`);
    }

    console.info(`Socket ${socketId} is closed`);
  };
}

server.on('connection', (socket) => {
  console.info('A new TCP connection is created on port', socket.localPort, socket._handle.fd);

  socket.on('close', getSocketCloseFn(socket._handle.fd));
});

server.listen(7690, () => {
  console.info('server listening at port 7690');
});

We also uses nginx as a reverse proxy with the following configuration -

upstream backend {
  server 127.0.0.1:7690;
  keepalive 8;  
  keepalive_timeout 3600;
 }
server {
  listen 9678;
  location / {
    proxy_pass http://backend;
    proxy_set_header Host $host;
    proxy_http_version 1.1;
    proxy_set_header Connection "";
  }
}

The logs of server are as follows -

A new TCP connection is created on port 7690 19
A new TCP connection is created on port 7690 20
Writing response to the socket 19
Writing response to the socket 20
Writing response to the socket 20
Writing response to the socket 20
Writing response to the socket 20
Writing response to the socket 20
Writing response to the socket 20
Writing response to the socket 20
Socket 19 is closed
Socket 20 is closed
A new TCP connection is created on port 7690 19
A new TCP connection is created on port 7690 20
Writing response to the socket 19
Writing response to the socket 20
Socket 20 is closed
Socket 19 is closed
A new TCP connection is created on port 7690 19
Writing response to the socket 19
Writing response to the socket 19
Writing response to the socket 19
Writing response to the socket 19
A new TCP connection is created on port 7690 20
Writing response to the socket 20
Writing response to the socket 20
Writing response to the socket 20
A new TCP connection is created on port 7690 21
Writing response to the socket 21
Socket 19 is closed
Socket 21 is closed
Socket 20 is closed

I am trying to understand even after configuring keepalive_timeout 3600; in nginx configuration why socket connection between nginx and nodejs server is closed. Shouldn't this be the case that TCP connection will be connected throughout 1h?