This is unfortunate behavior on timeout, and we've shared a workaround solution available using maps. There's a configuration example in this Gist: https://gist.github.com/thresheek/2fa6479ffb7aca710493.

We're also going to prioritize a complete fix in the product, and encourage your comments and input on this ticket: https://trac.nginx.org/nginx/ticket/488

Disclaimer: I work @ NGINX. Thanks, Owen