phpBB doesn't have good cookie handling?
Not for me, anyway. I've noticed that any threads I open in new windows don't get marked read properly, so I end up having to read all the new posts in a forum and then mark it read manually.
Even more annoying is that if you log out with posts unread, phpBB marks them all read on you. Poo!

If I had to guess, it seems maybe it's because phpBB post new-ness is based on the time you visit a forum index, rather than keeping track of posts themselves?

I don't know PHP, so I can't say for sure, just a guess on "observed behavior".
Peace & Luv, Liz