Notification is not sent when a comment is added in a sub-section of the topic one subscribed to.
This happens because the interface only allows you to subscribe to sections with level 2 headings, but we attempt to send notifications to subscribers of the closest heading, even if it's level 3 (or some other level).
For example, this problem would occur in discussions like this:
- …when another section has been merged into this one as a sub-section:
- …when an arbitrary section break is added, so that commenters don't have to scroll so far to click [edit source]: