How to debug page caching?
The X-Drupal-Cache-Tags header should help with this.  Here's how to enable it from this doc:


  You can debug cacheable responses (responses that implement this
  interface, which may be cached by Page Cache or Dynamic Page Cache) by
  setting the http.response.debug_cacheability_headers container
  parameter to true, in your services.yml. Followed by a container
  rebuild, which is necessary when changing a container parameter.
  
  That will cause Drupal to send X-Drupal-Cache-Tags and
  X-Drupal-Cache-Contexts headers.