Not sure if this is a known problem, but for me it sometimes happens (especially when coming back to a browser tab after a few hours) that the comment threads and the main post are from two entirely different posts. Since it probably matters, I’m using beehaw via the Brave browser.
I’ve had this happen in Jerboa a few times (the android lemmy client). I’m pretty sure it’s happened to me with comment threads on both beehaw and other instances, but definitely at least on beehaw.