Some users interpret that as implying you are subscribed and will get Email notifications.
Whereas that is the default setting and you are not subscribed, as has always been the case.
When you click to subscribe, the tick clears, implying you are not subscribed, but in fact you are subscribed
The similar Bookmark topic and Remove from bookmarks options behave much more rationally.
Its associated icon does not change and the legend is quite clear.
If the Subscribe/Unsubscribe topic had a similar unchanging icon, then the legend is clear enough, less confusing, and the action confirmed by the popup message.
I suspect the above behaviour has possibly been introduced since changing server or theme?