proxy_cache_bypass keeps serving old cached response if URL changes to non-cacheable

Under normal circumstances, when using proxy_cache_bypass nginx will fetch a fresh copy from upstream, and overwrite the cached response with the new one.

But if the URL changes from a cacheable to a non-cacheable response (for example to a 4xx response with Cache-Control: no-cache), then using proxy_cache_bypass will indeed serve a fresh copy from upstream, but it will leave the old copy in the cache.

Which means that everytime the URL is requested without triggering proxy_cache_bypass, it will keep serving the old cache.

I guess this is an intended behaviour, because proxy_cache_bypass only overwrites cached responses by saving a new one, and a no-cache response means there is nothing to save? Is this what's happening?

How can I solve this? I don't want to enable caching for 4xx responses...

I'm running nginx/1.14.2

proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=my-cache:70m max_size=28g inactive=1d;
proxy_temp_path /var/cache/nginx/tmp;
proxy_cache my-cache;
proxy_cache_key $remote_user$scheme$host$request_uri;

proxy_cache_use_stale error timeout invalid_header http_500 http_502 http_503 http_504 http_429;

proxy_cache_bypass $http_cache_control;

proxy_read_timeout 90;
add_header X-Cache-Status $upstream_cache_status;

etag off;

You can tell nginx to cache only 200, 301 and 302 responses by specifying

proxy_cache_valid 10m;

This tells nginx to cache valid responses for ten minutes.

This is the expected behavior. That's why they want you to use the paid version to get the proxy_cache_purge directive