Depends of reverse proxy how it talks with a backend. Varnish, if not Enterprise, doesn’t use SSL after termination and that’s why reverse proxy must send something like proxy_set_header X-Forwarded-Proto https;.
But good to know I have to go to hidden settings next time I use Varnish between Nginx and Discourse (plus something else, because otherwise it would be just stupid in my scale).
Or am I totally misunderstanding what forcing means now?