Nginx proxy_cache_background_update after cache expired

I'm trying to configure nginx to use proxy_cache_background_update, but it seems that after the expiration date, it still waits for a full transition back to the backend, returning MISS to X-Cache-Status. What am I doing MISSing?

I am using nginx 1.11.12 under ubuntu 14.04 running inside docker, but hopefully this is too verbose.

location ~ ^/?(\d+/[^/]+)?/?$
{
   expires 20s;

   proxy_cache app_cache;
   proxy_cache_lock on;

   proxy_cache_bypass $http_upgrade;

   proxy_pass http://172.17.0.2:5000;
   proxy_http_version 1.1;
   error_log    /nginxerror.log debug;

   add_header X-Cache-Status $upstream_cache_status;

   proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
   proxy_cache_background_update on;

   break;
}