Yes, upstream behaves as you would expect.

Right now Nginx proxy_valid is set to 1 second. After that second Nginx revalidates with upstream and upstream will respond with 304 if applicable.

I just do not want Nginx to serve from cache during that second. It always needs to revalidate.

Apache supports this as noted here by somebody else: https://stackoverflow.com/questions/41252208/nginx-cache-but-immediately-expire-revalidate-using-cache-control-public-s-ma