I believe you are referring to EasyDiscuss? It's actually not a bug but if you try to view a "category" in EasyDiscuss, you could actually subscribe to the category rather than site wide.
I just tested this locally and it seems like the subscription is working correctly as it should for both site subscription and category subscription. By the way it seems like your current easydiscuss version is not on latest version. Perhaps you can try to update your easydiscuss to latest version 3.2.9744 and see how it goes?
If the issue still persist kindly provide us with your Joomla backend and FTP access so we can check on this directly.