I have my Flarum installed in a sub-folder:
Base URL: https://myforum.com/forum
Installation path: /home/customer/www/myforum.com/public_html/forum
If I click on the timestamp of a post to reveal the URL link to that post, the path segment
forum is repeated, for example:
I'm not sure when that happened, maybe after 1.2.1 or maybe earlier, I haven't used that functionality for long but it's now broken.