This has been happening to me for AGES on my Win7 lappy. IIRC it didn't start IMMEDIATELY after the last major forum overhaul but shortly after.
Since then if I comment on something or even just click on a thread next time I click on the "First Unread" thingie it'll toss me to some semi random post (sometimes it'll be a post I make and get stuck there every time I click, sometimes it's someone else's post, sometimes it CHANGES the post then sticks to that... it's crazy).
I've mentioned it before and others said they were expereincing it too (this was like a year ago).
Now if I want to read new posts I click the last page button (or if only a one page thread just the title), Page Down and then scroll backwards to where I left off (kind of helpful because I scan the later posts on the way backward too).
It's pretty annoying though.
I use Firefox (most current version) and have some ad/script blockers going so maybe that's it. I know most people use Chrome these days but I still lurvs FF.
/blathering