When subscribed to a forum the notifications for new topics are coming in without the 'e' parameter present in the forum or topic links which needs to be there for certain processing by viewforum.php which might also be causing a need to visit the new topic and not just the forum to reset the "visited" flag (email states "No more notifications will be sent until you visit the forum.").

Notifications about replies seem to include the 'e' parameter for forum subscriptions.