Pending boundary inside async component never mounts result

[ottomated]

Describe the bug

An example as simple as this is broken in SvelteKit:

<script>
  await Promise.resolve();
</script>

<svelte:boundary>
  <div>Loaded</div>
  {#snippet pending()}
    Loading
  {/snippet}
</svelte:boundary>

The "Loaded" text never appears.

Reproduction

reproduction

works on sandbox

Logs

System Info

System:
    OS: Linux 6.13 Fedora Linux 43 (Forty Three Prerelease)
    CPU: (24) x64 AMD Ryzen 9 7900 12-Core Processor
    Memory: 16.89 GB / 30.48 GB
    Container: Yes
    Shell: 4.0.2 - /bin/fish
  Binaries:
    Node: 22.19.0 - ~/.local/share/pnpm/node
    npm: 10.9.3 - ~/.local/share/pnpm/npm
    pnpm: 10.18.0 - ~/.local/share/pnpm/pnpm
    bun: 1.2.20 - ~/.bun/bin/bun
  Browsers:
    Chrome: 141.0.7390.54
  npmPackages:
    @sveltejs/adapter-cloudflare: catalog: => 7.2.4 
    @sveltejs/kit: catalog: => 2.43.8 
    @sveltejs/vite-plugin-svelte: catalog: => 6.2.1 
    svelte: catalog: => 5.39.8 
    vite: catalog: => 7.1.9

Severity

blocking an upgrade

Additional Information

No response

[dummdidumm]

reproducible with local Svelte playground and SSR enabled