Ok, I see what happened. When posts are split from an old thread to make a new thread, each post inside is assigned a new post #. Nothing is deleted, they just all get new post numbers. But your acorn tip is referencing an old post # that no longer exists.
The problem then is that the acorn tip refers to a post that has been moved, but it doesn't know where it is. The best solution to prevent this from happening in the future is to update the acorn tip record with the new post # for each new post.
