I've been having the same issue. Windows 8.1 and Firefox. When I come to the site and scroll down to find the last post I had opened with new content after I read the topic and click back it takes me to a page or sometimes 2 pages back of where I was.
New! Dark mode!
Browsing Pinside at night? Getting tired of all the white? Switch to dark mode using the button in the top right (or CTRL-B)!