Webpack: -p flag and cssnano - broken stylesheet?

Question

webpack

Why does the file get corrupted when running webpack with the -p flag (in production) and the cssnano minifier?
This happens when I include 2 files from node_modules:

@import '~bootstrap/scss/bootstrap';
@import '~plyr/src/scss/plyr';

If I connect one thing, then everything is compressed, if both, then there are no plyr styles in the collected file at all ...
If I remove the -p flag when starting webpack, then everything works fine. What could be the problem?