I think this is a major bug
After a post is made, when someone reads the post they have 2 options. They can click on reply and their reply become part of the thread or they can post in the box at the bottom which is titled the same as the original post but is not a reply but a new post. If you click on the post in the new posts area it will not include the original post and can be very confusing as demonstarted by Eri's "what hoax" post.
This could explain why some have thought their posts disappeared.