cache POST request with Nginx

api.example.net.conf

upstream api_example_net {
  server api.example.net:4000;
  keepalive 600;
}

proxy_cache_path /var/cache/nginx/tag levels=1:2 keys_zone=tag:10m inactive=1d max_size=10g;

server {
  listen 80;
  server_name api.example.net;
  default_type application/json;

  access_log /var/log/nginx/api.example.net.access.log main;
  error_log /var/log/nginx/api.example.net.error.log;

  client_max_body_size 2m;
  client_body_buffer_size 2m;

  set $request_body_md5 "";
  rewrite_by_lua '
    ngx.req.read_body()
    local body = ngx.req.get_body_data()
    ngx.var.request_body_md5 = ngx.md5(body)
  ';

  location / {
    proxy_http_version 1.1;
    proxy_set_header Host $host;
    proxy_set_header Connection "";

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

    proxy_cache tag;
    proxy_cache_key "$uri|$query_string|$request_body_md5";
    proxy_cache_valid 200 1d;
    proxy_cache_methods POST;

    add_header X-Cache-Status $upstream_cache_status;

    proxy_pass http://api_example_net;
  }
}

i tried this on nginx/1.13.6.
the post request has some 5 fields.
Caching works but same request from different browser or even curl is taken as a new request and cached newly.