No CSS minification when using css.extract = false option

[lehni]

Version

3.0.0

Node and OS info

Node v9.11.2 / Yarn 1.7.0 / macOS 10.13.6

Steps to reproduce

When using Vue CLI 3 to create a library of components (vue-cli-service build --target lib) with the css.extract = false option, the CSS included in the minified JS file of the library (ending in umd.min.js) isn't minified.
When setting css.extract = true, the generated separate CSS file is minified.

What is expected?

The CSS that is "inlined" in the minified JS file should also be minified.

What is actually happening?

It is included as is, with lots of white-space, increasing file-size