Issue
Cloudflare does not respect the "Vary" header at all
Resolution
Achieving authenticated data personalisation every development team will create a Browser Cookie, be it PHP or Javascript. The generation of a Cookie is necessary because of the static HTTP protocol, and its inability to differentiate between authenticated and anonymous users access a website.
Acquia Cloud Edge caching will not recognise authenticated users custom HTML personalisation, and any request will return what cached content the CDN layer is storing. That action inhibits the ability of Drupal to return personalised HTML or JSON to the user.
To allow Acquia Edge to pass personalised content to a user, your development team needs to add a page rule according to the Cloudflare article, Caching HTML with Drupal.