Unread post flag keeps getting cleared

[GenePoole]

I don't know if I have a setting messed up or if it is just an anomaly of vBulletin that I have to live with, but the flag or whatever that marks a post as "unread" is getting cleared without actually reading the post.

Here's an example. I have a browser bookmark to my user CP. I click on that and it shows my subscribed forums. I'll browse through the new posts on one of my subscribed forums. I get distracted and come back 30 minutes or so then click on another subscribed forum. To my dismay, all the "unread" posts now show up as read.

As a counter example, I browse the forums at codeguru.com (using vBulletin 3.7.3--a little bit newer version than here) and don't have this problem. Posts remain unread until I read them, or manually mark the forum as read, but NEVER anytime else.

Any help? Comments?