I use email notifications to keep track of threads I’m interested in. Most of the time, if I click on the link in the thread to go to the new posts in a thread, it will take me back to some point earlier in the thread. It will stay on that point perpetually. For example, in one thread I’m participating in, it’s post 165.
It’s just a wonky calculation between cookie information and the board software and what is “new” versus what is “new to you” and … it’s complicated, and I’m often surprised when it does work.