Okay a bit more info to reproduce.
Make sure to use a topic with 15ish posts.
Click reply on the topic
Enter a line or two of text
Click topic title in reply window
Quickly scroll to bottom
Topic progress should be “off” (you see 14 of 20 instead of 20 of 20)
Click in reply text box, offset should definitely happen.
(I’m struggling to hit reply right now using this technique)