CSS is duplicated when used by a dynamically imported module

[lilnasy]

Reproduction project

https://stackblitz.com/edit/node-c8jpcemz?file=package.json

What the project is doing

There is a main entrypoint (the router) which dynamically imports components (the pages) that have styles attached to them via a side-effect CSS import statement.

What's going wrong

In addition to CSS chunks being emitted next to the module they were used by, there is non-split CSS chunk next to the main entry chunk.