On safari mobile (iOS 11.1..2) and whenever I follow an external link to another page, if I use the browser back function to return to the forums it automatically updates my current position in the thread to the next page.
So say I am on the second page of a thread, and the fifth comment has a link. I click on the link, read whatever, and then hit back to return to the forums. I will initially see the same comment (second page fifth comment), but then the page will update and it when it reloads I will now be looking at the first comment on the third page. At which point I have to go back a page and then find where I left off.
It doesn’t happen if the link opens a new tab. Switching to the new tab and returning to the forums tab has no effect and it stays on the same comment. It also doesn’t seem to happen if I have previously read the comment. So if open a thread and go back a page or two, clicking a link and then returning will stay on the comment with the link.
Posts
While you're waiting for an actual fix you can probably workaround this as follows.
Before opening the link, click the timestamp on the post (it's a permalink to that particular post). That way the URL for the page you're on will be to that specific post, so even if safari does reload when you go back it will reload at the same place.
e: sure enough though it's currently working like you describe on desktop as well, going back to the same page of the thread you just left if you use the back button even though it's using the latest post link, maybe it CAN tell that you're pressing the back button.
maybe I'm crazy!