My configuration looks like this:

  postcss({
    extract: true,
    minimize: process.env.NODE_ENV === 'production'
  })

https://rollupjs.org/guide/en/#augmentchunkhash
https://github.com/egoist/rollup-plugin-postcss/blob/master/src/index.js#L140
The augmentChunkHash hook is used to augment the hash of chunks (entry js file + its dependencies), not assets, so I don't understand why this is used. It's not even filtering which chunk gets its hash augmented.
This is causing all my js files to have new a hash on every build, even when nothing has changed.

The extracted stylesheet's hash should also not be based on the entry file's hash, it should have its own independent hash.

Removing the augmentChunkHash hook completely fixes first problem. rollup-plugin-postcss will still need to hash the stylesheet based on its content.