Had to log in to the server and do a global grep for the hostname on the file system and found an instance in a file called app.yml in a folder /var/discourse/containers. Apparently the hostname of the forum is hard-coded into the application instead of being configurable.
Edited the file, and it still didn’t work. Did some googling and ran /var/discourse/launcher rebuild app and after a few minutes of downtime it came back, and appears to be using the correct hostname now.