forward proxy config is causing "upstream server temporarily disabled while connecting to upstream" error

[Señor J Onion]

I want to set up nginx as a forward proxy - much like Squid might work.

This is my server block:

      server {
         listen 3128;
         server_name localhost;

         location / {
            resolver 8.8.8.8;
            proxy_pass http://$http_host$uri$is_args$args;
         }
      }

This is the curl command I use to test, and it works the first time, maybe even the second time.

    curl -s -D - -o /dev/null -x "http://localhost:3128" http://storage.googleapis.com/my.appspot.com/test.jpeg

The corresponding nginx access log is

    172.23.0.1 - - [26/Feb/2021:12:38:59 +0000] "GET http://storage.googleapis.com/my.appspot.com/test.jpeg HTTP/1.1" 200 2296040 "-" "curl/7.64.1" "-"

However - on repeated requests, I start getting these errors in my nginx logs (after say the 2nd or 3rd attempt)

    2021/02/26 12:39:49 [crit] 31#31: *4 connect() to [2c0f:fb50:4002:804::2010]:80 failed (99: Address not available) while connecting to upstream, client: 172.23.0.1, server: localhost, request: "GET http://storage.googleapis.com/omgimg.appspot.com/test.jpeg HTTP/1.1", upstream: "http://[2c0f:fb50:4002:804::2010]:80/my.appspot.com/test.jpeg", host: "storage.googleapis.com"
    2021/02/26 12:39:49 [warn] 31#31: *4 upstream server temporarily disabled while connecting to upstream, client: 172.23.0.1, server: localhost, request: "GET http://storage.googleapis.com/my.appspot.com/test.jpeg HTTP/1.1", upstream: "http://[2c0f:fb50:4002:804::2010]:80/my.appspot.com/test.jpeg", host: "storage.googleapis.com"


What might be causing these issues after just a handful of requests? (curl still fetches the URL fine)