Ok back on it. Looks like you are right.
So I tried to set DISCOURSE_S3_CDN_URL as a bucket subpath https://hot-objects.liiib.re/forum-beta-liiib-re and it is now working.
Do you think it would be reasonable to allow S3 CDN as a subdomain of S3 endpoint ?
In my case, which could be similar to other self-hosted setups, I’m not using an external CDN (nginx on top of minio can be considered as the CDN) so I don’t need to set it on another domain.
I actually setup the subdomain bucket because, I think, its required for discourse to manage uploads
Anyways it solved it and its working for me. Thanks for the help !