Error regression since 5.39.0

[vladshcherbin]

Describe the bug

There is quite significant difference in errors.

Async mode and a simple redirect in remote function on a page gives:

<= 5.38.10

✅ error is visible in console
✅ app is live after error
✅ error page shows error

>= 5.39.0

✅ error is visible in console
❌ app is dead after error
❌ no error page due to dead app

>= 5.39.1

❌ error is undefined in console
✅ app is live after error
❌ error page is not shown due to redirect

Reproduction

https://github.com/vladshcherbin/svelte-broken-error

Logs

System Info

System:
    OS: macOS 26.0
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.0.1
    npm: 11.3.0
    pnpm: 10.20.0
  Browsers:
    Chrome: 142.0.7444.60
    Safari: 26.0
  npmPackages:
    svelte: ^5.43.3 => 5.43.3

Severity

blocking an upgrade