[5.43.9] Async `{#if}` block failing

[sillvva]

Describe the bug

When I have a remote function that returns an array, the {:else if data.length} block is not rendering, even though the function always returns 2 items.

In my local dev environment, I'm getting:

Cannot read properties of undefined (reading 'length')

It happens with or without a boundary and with or without a pending snippet in the boundary.

In the REPL, you can see the fallback {:else} block rendering the data, even though it should never reach that block.

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-kfjqgvne?file=src%2Froutes%2Ftest.remote.ts,src%2Froutes%2F%2Bpage.svelte

Logs

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    svelte: ^5.43.10 => 5.43.10

Severity

blocking an upgrade