local `@import` statement isn't stripped when placed after `@source`

[ntjess]

Reproduction link or steps

https://stackblitz.com/edit/github-on9qlarn?file=dist%2Fstyle.css

1. Minimal package.json with @tsdown/css and tsdown installed
2. empty index.ts
3. style.css that imports from arbitrary content in ./theme.css:

@source "../index.ts";

@import "./theme.css";

4. tsdown.config.ts that makes ./src/assets/style.css an entrypoint / exposed to the bundler

What is expected?

According to https://tsdown.dev/options/css,

All imported CSS is bundled into a single output file with @import statements removed.

Meaning there should be no @import ./theme.css present in the bundled style.css.

What is actually happening?

The local import is still present, meaning the browser raises errors when actually importing the stylesheet:

/* contents of dist/style.css */
@source "../index.ts";

@import './theme.css';

Any additional comments?

If we swap the statement order (e.g., place @import before @source), everything works as expected and the import statement is not present in the output.