This sounds like a misconfiguration. I haven't used Cloudflare, but I've used other CDNs, and they need to generate a cache key with as few header values as possible to maximize the cache hit rate. The headers used in generating the cache key is configurable, so if you want to use the accept header in your application, then you're free to do so, but you need to tell Cloudflare that's an important part of your application.