When you search all pages within a prefix, the namespace input is misleading.
E.g. on https://en.wikipedia.org/w/index.php?search=foo&prefix=Wikipedia%3AVillage+pump&fulltext=1 it says that only pages in NS 0 are searched. You can also change the namespace selection, but no matter what you do, the search always searches the Wikipedia namespace, neither more nor less.
The namespace input should be hidden, deactivated or otherwise not misleading when performing a prefix search.