The request URL with the port, But response the next url without the port

[rikeitou-0191632]

Request URL: http://xxx.xxx.xxx.xxx:81/aaa

Response content: 
{
  "next": "http://xxx.xxx.xxx.xxx/aaa",
  "previous": "http://xxx.xxx.xxx.xxx/aaa",
  "current": "http://xxx.xxx.xxx.xxx/aaa"
}

Nginx

upstream django_server {
    server django:8001;
}

server {
    listen       81;
    location /aaa {
         proxy_pass http://django_server;
         ...
    }