I think I may have just seen this while reading (not posting) using Firefox 130.0 on macOS 14.6.1 on a 2018 Mac mini.
I was reading comment 24 of the “Finale’s Finale” thread.
As I was reading it, comment 25 was posted, and the screen immediately scrolled to put the new comment at the top of the screen, forcing me to scroll back to what I was reading.
This sounds like a bug Discourse. And since I haven’t seen this before, I’m assuming it’s from a recent update.