Describe the bug

I have the start project.
With one change: I added build: { sourcemap: true } in the config.

After building it has missed source maps for App.vue file.
Here is:

While in dev it is showed:

App.vue file script block has just only an import:

<script setup>
import HelloWorld from './components/HelloWorld.vue'
</script>

If I will add, for example:

console.log(123);

It starts works as expected:

Reproduction

yarn create @vitejs/app my-vue-app --template vue

and use the follow config (in vite.config.js):

export default defineConfig({
  plugins: [vue()],
  build: {
    sourcemap: true
  }
})

System Info

Node: 15.9.0
@vitejs/plugin-vue: 1.2.1
vite: 2.1.5

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