Actually I also encountered this recently in the context of the ActivityPub plugin while testing the ActivityPub integration between the Wordpress ActivityPub plugin and Discourse locally.
I’ve changed the category to Development here because I think it’s only a development issue with discourse/discourse and only manifests with a PHP remote (because of how the PHP request functions handle things or something like that).
I can’t recall exactly right now, but I think I came to the conclusion that this gets set elsewhere in production? I will try to remember tomorrow.