FYI, this problem exists on MF also and I have tested for it with three different browsers.
I have noticed that when a new topic is available, there is a little yellow folder next to the topic name on the left. The yellow folder meaning that it is new and unread.
When you click on the topic and it opens you can read the main (initial) posting and see (in threaded mode) where users have replied below... each unread reply showing the little yellow folder to the left.
When you click on a reply, the reply is displayed in the reading box/window/panel and the other replies are listed below only now all the little yellow folders are no longer yellow - as if they have all been read.
This is a little frustrating if there are many replies and you are not sure which you have read. I suspect the board was designed to retain the yellow folders for replies that have no yet been read.
I have found a work-around and that is to open the reply in a new tab then close that tab after reading the reply... going back to the original tab and opening the next reply in a new tab.
Can tech support look into this please?