I cannot reproduce this, but I don't doubt it happens for other users. Guess I am in the lucky bucket. Maybe Firefox users get an older version without some new "improvements". :)
Edit: Nevermind, it happens to me as well, I just never noticed.
I went to do a sanity check using your steps and unfortunately I missed the problem the first time around - when I go to a tweet thread, then a reply thread, then come back to the tweet thread, it is indeed scrolled all the way up to the top. So I was wrong, I am affected by the same issue, I just never realized it until I went looking for it. One positive I'm taking away from this is the realization that I use Twitter less intensively than I thought, because I wouldn't have noticed this without the explicit steps I don't think. :D
