I have a bunch of Nginx + PHP8.0-fpm installed in the north. When crawlers visit a page and the request contains the If-Modified-Since header, but the page has not changed, the site returns the "304 Not Modified" header. If the request protocol is HTTP / 2.0, everything works correctly, but if the request protocol is HTTP / 1.1, a warning appears in the Nginx logs:

[warn] 910 # 910: * 21961 upstream sent more data than specified in "Content-Length" header while reading upstream, client: 66.249.76.145, server: site.com, request: "GET /category/page-title.html HTTP / 1.1 ", upstream:" fastcgi: // unix: /var/www/php-fpm/12345.sock: ", host:"site.com"

The site engine does not give Nginx the Content-Length header for sure and there is no content. I tested this by making requests to the site with the If-Modified-Since header via CURL.

Why is Nginx showing this warning? Is this an Nginx bug?