Hi Dave,
I have noticed a "new behavior" which is I think what you are describing. Here is what happens to me with my forum "workflow:"
I log in to the forum, and click on "Show unread posts since last visit." I am "logged in" on the initial window. When I see a thread I want to read, the problem will appear if if I open the thread in a new window. In a new window the thread is now logged out in that window, and the trouble has started. Alternately, if I simply click the link to open the thread in the same logged in window instead of opening a new window, I can read the thread because I am logged in, but, I must remember to click the back button. Why? If I close the thread window, I need to log in again from scratch, and here is the real kicker, the number of "unread posts" gets reset, and I loose the ability to see the "unread posts" I have missed. I hope this description is clear, but this is what happens and it did not work in this manner before. Not sure of the timing of when this started, but would guess it is this year.
I use private windows always, and always clear everything when I surf, cookies, caches, and history.
So long for now, Tom