Oh, I see now. It was a user who tried to be helpful by adding hashtags to his reply, not realizing that Groups.io does not track which message is a reply to which but regards any edits to the subject line as the start of a new thread. I'll just wait for the thread to fizzle out and then merge the topics.
Say, as a matter of interest, does it simply involve too much processing power for Groups.io to check the body of incoming messages for previous messages' message IDs (in the footers), to determine which message that message is a reply to, or at least which topic it belongs to? Users "helpfully" (and in good faith) improving subject lines is something that I regularly encounter.