Reactivity broken for {@const} after async boundary on navigation (regression after 5.43.0)

[yayza]

Describe the bug

{@const} in child component loses reactivity after navigation when parent uses svelte:boundary with await

eg. in parent

<svelte:boundary>
        {@const data = await getPayments(params)
        <TestComponent value={(data) => JSON.stringify(data.rows)} arg={data} />
</svelte:boundary>

eg. child component

<script lang="ts">
    type Props = {
        value: string | number | ((arg: any) => any);
        arg?: any;
    };

    let { value, arg = {} }: Props = $props();
</script>


{#if typeof value === "string" || typeof value === "number"}
     {value}
{:else if value instanceof Function}
    {@const result = value(arg)}
    {result}
{/if}

Workarounds that fix it:

• Adding 100ms delay to async function 😁
• move const to the script tag and $derive it from the props
• force read the {@const result} in the block , eg. console.logging, or result.toString(), etc.
• downgrading to 5.43.0

Reproduction

https://github.com/yayza/async-const-test

1. on initial load, paginate, everything sync correctly
2. click the button to navigate to some other page
3. go back
4. now paginate again and notice the value in the child doesn't change, the remote data, and the child's props are reactive, but result doesn't re-evaulate

Logs

Nothing logs

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (24) x64 AMD Ryzen 9 9900X 12-Core Processor
    Memory: 5.22 GB / 31.08 GB
  Binaries:
    Node: 24.6.0 - C:\nvm4w\nodejs\node.EXE
    npm: 11.5.1 - C:\nvm4w\nodejs\npm.CMD
    pnpm: 10.15.0 - C:\nvm4w\nodejs\pnpm.CMD
    Deno: 2.5.6 - C:\Users\admin\AppData\Local\Microsoft\WinGet\Packages\DenoLand.Deno_Microsoft.Winget.Source_8wekyb3d8bbwe\deno.EXE
  Browsers:
    Edge: Chromium (140.0.3485.54)
    Firefox Developer Edition: 145.0 - C:\Program Files\Firefox Developer Edition\firefox.exe
  npmPackages:
    svelte: ^5.45.6 => 5.45.6

Severity

annoyance