Cannot "mark" thread as read after changing last_post_date


I'm changing the last_post_date of selected threads to the current time (bumping them):

$this->_updateThreadsBulk($threads, $forums, array('last_post_date' => time()), 'threadUP');
The code works. But then the selected threads will always be marked as unread, no matter how many times I open them. They can only be marked as read if I mark all forum as read.

Any idea why this is happening?

Thank you