I'm not sure if you already ran into this (or ever will), but working with self-references is a bit different from working with normal associations. From the name of your association, I assume you try to use this self-reference to point to the child folder from the parent folder and a parent can have multiple child folders, but every child folder references to one parent folder (many-to-one or Reference Default).
If you would make a page with one parent folder and you would want to show the child folders over this association, you wouldn't be able to with Data source Type Association. You would need to retrieve the child folders with this Xpath:
[MyProj.ParentFolder_ChildFolder = '[%CurrentObject%]']
Good luck with your project!
Like I thought, it was something simple.