When setting read tracking on a forum to 'On', according to the documentation it should force read tracking for that forum. However there seems to be a bug where tracking will not be turned on unless the user has enabled forum tracking on in their profile settings.

To reproduce: create a forum with read tracking set to 'On'. View the forum as a user without the 'forum tracking' set in their profile, and the tracking information will not appear.

In mod/forum/lib.php, function forum_tp_can_track_forums():
The following logic seems to be the problem:

return ($forumforced || $forumallows) && !empty($user->trackforums);

because this requires $user->trackforums to be set regardless of $forumforced.

In 1.9.0 the logic was different, and the forum tracking worked correctly:
return ($isauser && ($forumforced || ($forumallows && !empty($user->trackforums))));

So between 1.9.0 and 1.9.1 the behaviour of the read tracking setting has changed.