Formerly, when you were subscribed to a thread, if you clicked Thread Tools you'd get Unsubscribe From Thread as an option.
Now you get Subscribe to Thread, and clicking it doesn't change that.
I suspect (but don't know for sure) that part of what's wrong is that posting to a thread subscribes you (as always and correctly), but doesn't toggle the "is he subscribed" switch in the part that determines the menu options. So the system treats you as subscribed (correct) but gives you the menu options for someone who isn't subscribed, i.e. the option to subscribe.
Either that or the whole mechanism just isn't working at all. There are a number of threads (especially from CE&P) that have drifted far enough from topic that I'm no longer interested in participating, but I can't get them out of my Subscribed Threads queue!