nginx reverse proxy - doesn't close upstream connection when HTTPS client disconnects

Question

I have a server which needs to know when the client closes the connection. The server is behind an nginx reverse proxy. Everything works fine over HTTP, but as soon as I turn SSL on, nginx seems to hold the upstream connection open for a full minute after the HTTPS client has disconnected.

How can I get nginx to close the upstream connection when the client disconnects?

Here's my nginx config:

ssl_session_cache off;
server {
    listen 443;
    ssl on;
    ssl_certificate server.crt;
    ssl_certificate_key server.key;
    location /find {
      proxy_pass http://my_upstream;
      proxy_ignore_client_abort off;
      proxy_buffering off;
    }
  }