Two issues I have seen lately on mobile.
First, when I reply to someone my screen/page jumps towards the top of the page. I have to scroll down to the text box.
Second, my last read comment is not always keeping place. This mostly happens when a new page of comments is started. For example, I will go back to a thread, and instead of showing last comment, it jumps to the top. I then read the new comments, and then click on to the new page of comments. Hope this makes sense.