This is just how it works. The thread subject gets the title of the first post but posts can allow different titles on the same thread so there is nothing to uppdate the thread subject by editing a post except moderators can edit the thread, change the title or move the thread to a different forum. If you make a mistake in the title of the first post on the thread the easiest thing to do is just delete it and repost it with the correct title. So I don't see it as a bug, I see it more as a feature request that if there is only one post and the title is edited it could be made to update the thread title, but once there is more than one post editing a post should not affect the thread title. I'll add a note about it in our project tracker.