“Posts page” doesn’t list subpages

The page selected as “Posts page” doesn’t seem list subpages.

On my site I am working on there is subpages under Concept that are listed but there is also subpages under News (which is the selected “Posts page”) and those subpages don’t list.

If I unselect News as the “Posts page” the subpages list.

Why is that – and more important, what could be the solution?