I don’t recall where I read this but as I understand it Cloudflare ignores Vary: User-Agent because (when exposed to the general public) it’s effectively the sa

I don’t recall where I read this but as I understand it Cloudflare ignores Vary: User-Agent because (when exposed to the general public) it’s effectively the same as “never cache this page,” due to the wide variety of user agents and the amount of detail they pack into their user-agent string. It’s basically a very big hammer that (a) would massively increase the load on both Cloudflare’s cache and the backing server, and (b) almost never does what the server owner actually wants (which is usually “send a different variant on this page to IE users”).