This might require discourse’s force_https setting but if I enable this, it could break this forums’s .onion. With some server config changes, I managed to enable force_https and yet not break this forums’s .onion. I hope it does not have any bad site effects discovered later. Does this help?
What you could also try is making an account at https://meta.discourse.org and see if you can reproduce the issue there too. That would actually be good if the bug happens there too because then it’s unspecific to Whonix forums and maybe a bug that upstream could fix more easily.
(Severing discourse on two different domain names, let alone https + onion is already unsupported by upstream. And this was difficult to achieve. → Can I have two domains pointing towards the same IP address without redirect? - support - Discourse Meta)