I have noticed that if a forum lights up and you visit it and there were more than one post lit up in it...and you click on one of them, the forum's light goes out...I think the forum believes that if you've been there, you've checked ALL the relevant posts...
If you then want to read the others which you haven't read when visiting the forum while the light was on, you need to remember which ones they were...because they will be shown as having been read... At least that is in my experience.
The only way to avoid that is after you have read one post in a specific forum, (and there are more posts to read), is to click on the previous page on your screen and go back to the originally lit up forum and then pick the next post from there... It's a shlep but well...we just have to keep our eyes peeled.
On the other hand, as you say it may be one of those 'enigmas' that from time to time pop up.... This site is after all 'enchanted'...Heh..so anything is possible here...
love and rainbows
Raka