Eleventy addPassthroughCopy for Javascript folder doesn't copy files

Probably I'm missing something completely obvious. I tried to pass through my main.js file but nothing seems to work.

module.exports = function (eleventyConfig) {
    eleventyConfig.addPassthroughCopy("./src/assets/js");
    eleventyConfig.addWatchTarget("./src/assets/");
  
    return {
      dir: {
        input: "src",
        output: "public",
      },
    };
  };

My folder structure is set up as follows

📦eleventy-project
 ┣ 📂src
 ┃ ┣ 📂_includes
 ┃ ┃ ┗ 📜base.njk
 ┃ ┣ 📂assets
 ┃ ┃ ┗ 📂js
 ┃ ┃   ┗ 📜main.js
 ┃ ┗ 📜index.njk
 ┣ 📜.eleventy.js
 ┣ 📜package-lock.json
 ┗ 📜package.json

Everything else works. The target watch works, input and output directories work but the passthroughCopy doesn't do anything.

Does anybody see my mistake?

Okay, stupid me. The passthrough seems only to take effect on build not on serve (at least in 11ty 2.0). Site works, it just copies the files only on build.