Named form actions do not work with Vercel ISR.

Describe the bug

A named form action returns a 500 Internal Error when served from Vercel with ISR.

Reproduction

Steps:

1. Clone repo: https://github.com/Leftium/kit-demos/tree/isr-actions
2. Deploy to Vercel
3. Load / (root path): https://kit-demos-git-isr-actions-leftium.vercel.app/
4. Click "Submit" button

Expected:

• "undefined" turns to "true" (the property form.success from return value of form action.)

Actual:

• 500 Internal Error
• Vercel log may show error message like "cannot find default form action."

Logs

No response

System Info

System:
    OS: Windows 11 10.0.22621
    CPU: (8) x64 Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz
    Memory: 8.99 GB / 23.86 GB
  Binaries:
    Node: 18.18.2 - V:\scoop\apps\nodejs-lts\current\node.EXE
    npm: 9.8.1 - V:\scoop\apps\nodejs-lts\current\npm.CMD
    pnpm: 8.11.0 - V:\pnpm-home\pnpm.CMD
  Browsers:
    Edge: Chromium (120.0.2210.61)
    Internet Explorer: 11.0.22621.1
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.1.1
    @sveltejs/kit: ^1.20.4 => 1.30.3
    svelte: ^4.0.5 => 4.2.8
    vite: ^4.4.2 => 4.5.1

Severity

serious, but I can work around it

Additional Information

• The error does not happen if the default action is used: https://kit-demos-git-isr-actions-leftium.vercel.app/with-isr-default-action
• The error does not happen if ISR is not used: https://kit-demos-git-isr-actions-leftium.vercel.app/without-isr

So the work-arounds are to not use named form actions or not use ISR.

Reproduce the issue using the linked kit-demos isr-actions branch and compare the named-action, default-action, and non-ISR pages. Then trace SvelteKit's Vercel ISR form-action handling; done means the named action returns form.success without a 500 error when deployed to Vercel.