PostCSS plugin xxx requires PostCSS 8

I'm getting the following error:

[!] (plugin postcss) Error: PostCSS plugin postcss-nested requires PostCSS 8.
Migration guide for end-users:
https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users

But the thing is I'm already using PostCSS 8 and the latest version of rollup-plugin-postcss. So everything the [Migration Guide](https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users) recommends:

```
"postcss": "^8.3.2",
"postcss-import": "^14.0.2",
"postcss-nested": "^5.0.5",
"rollup": "^2.44.0",
"rollup-plugin-postcss": "^4.0.0",
```
Any ideas why this error still appears? This happens with various PostCSS plugins.