I created a small next.js demo project with the following css code:

.page {
  button {
    color: green;
  }
}

Stackblitz without postcss-nesting
without postcss-nesting .page { button { .. } } is not pure:

however the same css code works once you add postcss-nesting

Stackblitz with postcss-nesting

I took a look and found out the reason:

without postcss-nesting the selector is button (which is not pure)
with postcss-nesting the selector is .page button (which IS pure according to postcss-modules-local-by-default)

To proof that I added a new passing test case without modifying the logic:

 {
    name: "consider global inside local as pure",
    input: ".foo button { a_value: some-value; }",
    options: { mode: "pure" },
    expected: ":local(.foo) button { a_value: some-value; }",
  },