Update from from 2.20.1 to 2.20.6 breaks form

[nitriques]

Describe the bug

Forms handled with "actions" are broken after the update to 2.20.6

Reproduction

Create a form usign the action pattern in previous kit versions: it work.
In 2.20.6, it's not.

Logs

See the console: it's a JSON.parse error in client.js

System Info

System:
    OS: macOS 15.1.1
    CPU: (8) arm64 Apple M1
    Memory: 97.44 MB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 22.12.0 - ~/.nvm/versions/node/v22.12.0/bin/node
    npm: 10.9.0 - ~/.nvm/versions/node/v22.12.0/bin/npm
  Browsers:
    Chrome: 136.0.7103.114
    Safari: 18.1.1
  npmPackages:
    @sveltejs/adapter-vercel: 5.6.3 => 5.7.2 
    @sveltejs/kit: ^2.20.1 => 2.20.6 
    @sveltejs/vite-plugin-svelte: 5.0.3 => 5.0.3 
    svelte: 5.23.2 => 5.23.2 

It's borken on vercel too

Severity

blocking an upgrade

Additional Information

Thanks for you hard work

[dummdidumm]

Please provide a minimal reproduction in the form of a github repository or stackblitz link

[elliott-with-the-longest-name-on-github]

closing dead issue. if you're still experiencing this problem, feel free to reopen with a reproduction we can use to diagnose!