Imported css is not transformed

[ycmjason]

Minimal reproduction
https://github.com/ycmjason/vite-issue-imported-css-not-transformed

The same css code get transformed differently in the "@import"ed css.

index.css

@import './import-me.css';

.a {
  &__nest {
    display: block;
  }
}

import-me.css

.b {
  &__nest {
    display: block;
  }
}

The CSS as I inspected becomes (formatted)

.b {
  &__nest {
    display: block;
  }
}
.a__nest {
  display: block;
}

As we can see import-me.css presumably did not go through postcss.