Can you try to count how many posts are above the one you are opening and check if it's the same amount when your return?
It could be that the browser remembers the viewport position but its content was changed (new posts) when you were inside a post.
Hmm static this time. Maybe that's it?
That might be the reason why the position of the viewport wouldn't change when its content is updated (dynamically pushed down by new posts).
(post is archived)