Vite plugin for SvelteKit: Generated files aren't added to the build output - vite

Question

I am currently working on another plugin, that should in the end generate the webmanifest and all images and splash screens needed for a PWA (minus the service worker). I am planning on making this a plugin for vite (rollup), with a special focus on sveltekit, because that's where I plan on using it.
I currently have this setup as a package that exports both mjs and cjs, and should for all I know have a working version to test with. Sadly, the output emitted using this.emitFiles doesn't appear in the build output, even though prior function returns an assetId that resolves to a URL.
Code
index.ts
import { Plugin } from 'vite'
import { PluginOptions } from './types.js'
import { readFileSync } from 'fs'
import { generateResizedWebpIcon, generateResizedJpegIcon } from './utils.js';
export default (options: PluginOptions): Plugin => {
const iconResolutions = [16, 48, 128, 512]
return {
name: 'vite-plugin-pwa',
async transformIndexHtml() {
// add images and manifest to build output
// generate icons and emit them, store the urls
const icon = readFileSync(options.image.src)
let icons = await Promise.all(iconResolutions.map(async res => {
const resolveID = this.emitFile({
type: 'asset',
name: `icon-${res}x${res}.webp`,
source: await generateResizedWebpIcon({...})
})
return {
type: 'image/webp',
sizes: `${res}x${res}`,
src: this.getFileName(resolveID)
}
}, this))
if (options.image.output?.jpeg) {
icons.push(...await Promise.all(iconResolutions.map(async res => {
const resolveID = this.emitFile({
type: 'asset',
name: `icon-${res}x${res}.jpeg`,
source: await generateResizedJpegIcon({...})
})
return {
type: 'image/jpeg',
sizes: `${res}x${res}`,
src: this.getFileName(resolveID)
}
}, this)))
}
const packageInfo = JSON.parse(readFileSync('package.json').toString())
const manifest = {
name: packageInfo.name || 'name',
description: packageInfo.description || 'description',
...options.manifest || {},
icons
};
const manifestUrl = this.getFileName(
this.emitFile({
type: 'asset',
name: 'manifest.json',
source: Buffer.from(JSON.stringify({
manifest
}))
})
)
// generate manifest with icons, save the url
// generate apple splashes and emit them, save the urls
// add links to manifest and apple meta tags
return [
{
tag: 'link',
attrs: {
rel: 'manifest',
href: manifestUrl
},
injectTo: 'head'
}
]
},
}
}
In this example, the <link rel="manifest" href="_app/manifest.webmanifest"> turns up in the html and chrome tries to fetch it. But the server returns a 404 Not Found code. It appears vite emits the file, but it is somehow overwritten by the sveltekit build process?
Does anyone know how to make this emit a file that also turns up in the final build output?