Confirmed.
It might stay broken for a long time.
Discourse upstream does not mind much about Tor / Tor Browser:
- discourse integration - Change Whonix forum software to discourse - #41 by nurmagoz
- discourse integration - Change Whonix forum software to discourse - #41 by nurmagoz
Multiple domain names are unsupported:
- How to add a second domain name? - Support - Discourse Meta
- How to allow multiple hostnames to login? - Support - Discourse Meta
- Can I have two domains pointing towards the same IP address without redirect? - Support - Discourse Meta
- Forcing hostname has broken support for Onion Services - Support - Discourse Meta
Developer information.
Discourse introduced this issue in version 3.3.3. (Commits · discourse/discourse · GitHub)