Okay, so I've done a lot of testing with it and
a) wherever you get to in the topic, it is marking it as read up to that point.
b) the 'new' image next to a topic will always be there unless you go to the last page of the topic. I did test this on my account, another account and fresh install and even on the official SMF forum, it only goes away if you go to the last page and it does it on every single thread for me. It does indeed seem to be the default behaviour though I don't think it used to be like this. I'm sure if you went to page one, it would mark the whole topic as read. But it's happening on the official forum so it's not an issue here. I don't think it's an issue with any of their updates either. It's a bit of a mystery.
c) I logged into your account Shasvre and it does mark it as read if you go to the last page. It does it on every thread every time. I can't really replicate the problem if it still isn't doing anything for you. I thought maybe it could be a browser, but I tested it in those too and it works as intended.