strange nginx behaviour with error_page + proxy_pass

Explanation.txt

I expected the first configuration below, simple.conf, to work just fine.

But it doesn't. My app server receives the second hit with the same URI as the first hit.

On the second config, wtf.conf, I keep the original URL on a variable to use in the error redirect.

My question is this: why doesn't the first simple.conf configuration works?

simple.conf

server {
  server_name my.site;
  listen      80;

  location / {
    proxy_pass http://127.0.0.1:5000;
    proxy_intercept_errors on;
  }

  error_page 404 /errors/404;
  location /errors/ {
    proxy_pass http://127.0.0.1:5000;
    internal;
  }
}

wtf.conf

server {
  server_name my.site;
  listen      80;

  location / {
    ## Keep copy of original URL
    set $orig_uri $uri$is_args$args;

    proxy_pass http://127.0.0.1:5000;
    proxy_intercept_errors on;
  }

  error_page 404 /errors/404;
  location /errors/ {
    rewrite  . /errors/404$orig_uri break;
    proxy_pass http://127.0.0.1:5000;
    internal;
  }
}