The software we use (Discourse) gets upgraded from time to time and sometimes features change when that happens. I see in the Discourse release notes that there has been some discussion of the Back Button being buggy in certain situations. So they might have yanked it, I’m not sure, but I just contacted them to ask.
Have tried using your browser’s back button, or is that problematic? Worth trying that in the mean time.
I’ll report back when I have an answer about this.