@SB
Editing the last post is where the problem lies. If you make a reply to someone’s post, you are the last one in the thread. Then you suddenly realized you spelled led like “lead”, so you edit the post and fix the “lead” to led and hit save. Now the thread goes back to the top and there’s no new reply’s.
Just wasn’t sure if it was clear as to how this was happening.
If it can’t be fixed then that’s fine.
It’s been that way as long as I remember and we are all still here, so no biggie. 
It’s not very nice to be abusing it as a free bump though especially if its your on thread.