Steps to reproduce: (this is how I found it. you probably want to use your test server.)

1. Go to https://moodle.org/mod/forum/view.php?f=1048
2. Verify that you are not subscribed to the forum as a whole.
3. Reply to one of the discussions there, chooseing to subscribe to the discussion.
4. Wait for the email to arrive.
5. Note that it says "Unsubscribe from this forum: https://moodle.org/mod/forum/subscribe.php?id=1048" near the end.
6. Click that link.
7. Note that you are now on a page saying "Do you really want to subscribe to forum 'Security and privacy'?"

That is wrong.