modules true and postcss-custom-properties doesn't seem to transform everything

Hi,

So I use postcss-custom-properties with css modules (`modules: true`) and have the postcss-custom-properties plugin and set it to `preserve: false` to that my `var(--something)` will not be kept in the css.

I seems to work for some of my css but not all as some cases doesn't seem to be transformed at all.

I've set up a repository here:
https://github.com/badtant/test

1. yarn install
2. yarn build:prod
3. search for `var(--` in the dist folder and there will be hits but I dont expect that

Not sure if this is an issue with rollup-plugin-postcss, postcss-custom-properties or somewhere else. It seems it's the parts that I compose from a different file that isn't transformed.

Thanks