So I've noticed this a few times recently. It doesn't happen all the time but a few times I thought there were multiple threads with unread posts but after reading one the others were gone. I have been paying more attention and noticed when I click on one thread to take me to the first unread post, it can mark other threads in the same subforum as read. I have not noticed this happening if I just click to the last page or click on the title, only if I use the "first unread post" button.
I am using Opera on a desktop, but I think it has happened on my phone (Android OS, using the built in Chrome browser).