Okay what I did was entered the app, then moved all of the assets in /var/www/discourse/public into a subdirectory called forum, then I moved back images/ and uploads/ and it all works great.
I think the issue is that the subdirectory got called for some assets, but not others.
I think this might cause an issue when I update Discourse next, but hopefully not.
I know I’m probably going full retard but I couldn’t find any other way.