Plugins not evaluating locally imported style sheets

I have the following `postcss.config.js`

```js
module.exports = {
plugins: [
require('postcss-import')(),
require('tailwindcss'),
require('autoprefixer'),
]
};
```

And the following `app.css`:

```css
/* Import Tailwind as Global Utils */

@import 'tailwindcss/base';
@import 'tailwindcss/components';
@import 'tailwindcss/utilities';
@import "./custom.css";
```

And `custom.css`:

```css
.hero {
background-image: url("../images/hero.jpg");
}
```

And the following `rollup` config:

```js
postcss({
extract: 'public/dist/app.css',
plugins: [
url({
url: 'inline'
})
]
})
```

Now if I move the `.hero` class into my `app.css` it works as expected and the background is inlined, however while it is in `custom.css` the code is not evaluated.

Cheers.