Drupal (or perhaps Apache) delivers wrong page
"The apache server is setup with some reverse proxy..." then you have to provide the correct response headers in an event subscribers, see How to embed drupal content in other sites (remove X-Frame-Options: SAMEORIGIN)? how to set response headers.

Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control

And uninstall the module Internal Page Cache, because this built in reverse proxy doesn't respect Cache-Control headers and you don't need it, because you already cache outside of Drupal, see PageCache caching uncacheable responses (violating HTTP/1.0 spec) + D8 intentionally disabling HTTP/1.0 proxies = WTF