I need help understanding the forum software behavior.

Can you describe more of the specific conditions around this issue?

You see, vBulletin does not have a permanent "read versus unread" thread capability, where threads stay marked as unread forever until you actually read them. Instead, it uses the concept of the member's last visit versus the last activity in a thread to show if it is read or unread by you.

Threads only show as unread if: 1. a post was made to it after your last visit time online on the forum, 2. during your current session, you haven't read the last page of a thread, after the last post was made to it, 3. you haven't used either the "Mark This Forum Read" (for a single forum), or the "Mark Forums Read" (for all forums), and 4. you have not timed out your session in the forum (15 minutes of inactivity).

The most common reason that threads you have not yet read show as if they were read (ie. no longer bolded) is that you have timed out your session, therefore your last visit time (shown in the upper right corner of every page above the private message stats) has updated to a more recent time. Basically, if you go idle on the forum for 15 minutes, your session resets and you lose all the unread thread markers.

Say it has been 5 days since your last visit, and there are a hundred unread threads marked when you start your session here today, if during the time you are reading threads you pause and don't click any forum link for 15 minutes, your session times out and you lose all those unread thread markers. (The image below shows my session here and the ten-forward sub-forum. Notice my last visit is 6:55pm and then look at the times in the ten-forward threads. "good game" was last posted at 6:53pm, which is before my last visit, so the forum thinks I read that thread when I come back online. But, the three threads above that one all had posts after 6:55pm, so they appear new (bold) to me.)

This is why I created the feature you show in your image. That set of boxes allows you to reset your last visit to be anything you want. Using the above example of a person having been away for 5 days, starting to read threads but then times out so they lose all the markers... they could use that User CP option to set their session back to the same 5 day mark again. (Notice the two check box options there. If you want to save the list of actual threads you read during the session, but still set the time back to recapture the other unread threads, then check that first box. The second option preserves the forums you've marked as read.)

Unfortunately, session management in vBulletin is very complex and trying to code a "sticky unread thread" feature into it would be terribly complex, so instead I added that session reset feature to let people get back their previous session times, or set any time range they wish, whenever they wish. Want to see all the "new threads" in the past month, then set your session back 30 days. Just want the last hour, then set it just 1 hour back.

FYI - That feature is located in the User CP > Edit Options screen for members.

 

That User CP option is only effective for your current session. After you logoff the forum, or timeout from inactivity, your last visit gets set (in the database) to the last real click you made on the forum. And when you come back, whether an hour later or a year later, the forum will show that last real click as your last visit time. And of course, threads that have new posts made to them since that specific moment in time will show as new for you when you come back.

You only really need to use that User CP feature if you've timed out prematurely and want to "set the clock back" in order to get back the unread thread markers. Or, if you want to experiment and play with different time intervals like I've mentioned... a month, a year, or whatever.