Likely what happened is someone made a reply to the thread that was on a completely different subject (AKA threadjacking) and robtillaart split that reply off into its own thread. I can't say exactly which post(s) might have been split off but since you were subscribed to that thread you should be able to review you email inbox to see which replies came in since the last reply on the current thread at 2018-02-27 to see what was split off.
It would be nice if the forum software would add links to both parts of the split topic rather than only one. Also, clearly that {UNSUBSCRIBELINK} think is a bug. That should be a link to unsubscribe from the thread.