remix run not executing dynamic route file

Here is a stackblitz with an example of the problem.

I have my setup like this:

I would expect a url like /posts/something to execute the $slug.tsx file but it is never called and instead a 404 Not found is returned.

This is my first time using remix but I am not sure what I am doing wrong.

I found the answer in this post.

All remix's docs are out of date or at least irrelevant with the new scheme.