Streaming Promises on a +page.server file misbehave in a built environment

Describe the bug

Streamed Promises work correctly in the development environment (npm run dev), but block the page load in a built environment (For example npm run build && npm run preview)

I'm using the latest version of sveltekit

Reproduction

• Install a new sveltekit app npm create svelte@latest my-app
• Install dependencies npm install
• Create a src/routes/+page.svelte file with the example from the SvelteKit docs

<script>
  /** @type {import('./$types').PageData} */  
  export let data;
</script>

<p>
  one: {data.one}
</p>
<p>
  two: {data.two}
</p>
<p>
  three:
  {#await data.streamed.three}
    Loading...
  {:then value}
    {value}
  {:catch error}
    {error.message}
  {/await}
</p>

• Create a src/routes/+page.server.js with the example in the SvelteKit docs

/** @type {import('./$types').PageServerLoad} */
export function load() {
  return {
    one: Promise.resolve(1),
    two: Promise.resolve(2),
    streamed: {
      three: new Promise((fulfil) => {
        setTimeout(() => {
          fulfil(3)
        }, 1000);
      })
    }
  };
}

• Run the dev environment npm run dev and load the dev page at http://localhost:5174/ (adjust the port if needed)
  Enjoy the "loading..." message for a second and then enjoy the final "3"
• Now build the app and preview it, npm run build && npm run preview and load the page at http://localhost:4173/ (adjust the port if needed)
  The page blocks for a second, shows "loading..." for an instant and then shows the final "3"

Logs

No response

System Info

System:
    OS: Linux 6.1 Fedora Linux 37 (Workstation Edition)
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
    Memory: 4.74 GB / 15.35 GB
    Container: Yes
    Shell: 5.2.15 - /bin/bash
  Binaries:
    Node: 18.15.0 - /usr/bin/node
    npm: 9.5.0 - /usr/bin/npm
  Browsers:
    Firefox: 111.0
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.0.0 
    @sveltejs/kit: ^1.5.0 => 1.14.0 
    svelte: ^3.54.0 => 3.57.0 
    vite: ^4.2.0 => 4.2.1

Severity

annoyance

Additional Information

No response

Reproduce the issue with src/routes/+page.svelte and src/routes/+page.server.js using npm run dev, then npm run build && npm run preview. Compare the loading behavior between development and the built preview; done means the streamed promise renders its loading state before resolving in both environments.