Yep, it's completely jacked. Very annoying. Of course, back in Twitter also randomly loses your place, too. I guess it's one of those "we're a big deal site now" checklist items: Fuck up the back button.
Oh well even HN will always mess up your scroll position when hitting back so it goes
But, taking it as true (for some arcane reason), then perhaps they should change the design of their site so that popping away from the timeline isn't a part of the normal usage flow!
Specifically, if you want to expand the conversation under a tweet, you tap it, which takes you to a new page; when you're done, you want to continue reading your timeline. If making the back button can't be made reliable, then it seems a design that simply expands the thread off to the right or inline below or in a popup, without leaving the timeline page, would work much better.
I think I found the problem.