This is related to MDL-47909 but different. I've noticed it when I posted a message to one of the moodle.org forums.

To reproduce:
1. Make sure your profile preference is "DON'T subscribe me automatically to forums"
2. Subscribe to one of the forums.
3. You will be receiving new messages from that forum (as expected)
4. Reply to one of the discussions - when posting the message, the default will be "I don't want to be notified of new posts in this discussion"
5. As a result, you end up subscribed to the whole forum EXCEPT the one discussion in which you've taken active part

This is counter-intuitive. Even though my profile preference is not to be automatically subscribed to forums, I explicitly chose to subscribe to this particular forum, so when I post a reply in one of the discussions in that forum, I should still be subscribed to it (by default).