Invalid field values in client-side enhanced form callback #16359

Describe the bug

Field values read in a remoteForm.enhance() callback may be (a) misaligned with the defined type (e.g. returning undefined instead of the typed non-nullable string[]), and (b) out of date when fields were controlled programmatically.

Consequently:

• Calling myForm.fields.myIds.map(...) may break at runtime. TS may have typed myIds as string[], but at runtime the value may resolve to undefined, and calling .map() will throw an uncaught `Uncaught TypeError.
• Client-side enhanced form logic that relies on field values via myForm.fields may not read the correct values.

Reproduction

Repro along with demo instructions:

https://stackblitz.com/edit/sveltekit-enhanced-form-missing-client-values?file=md!README.md

See

• Instructions: README.md
• Code: src/routes/+page.svelte & src/routes/form.remote.ts

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: 22.22.3 - /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:
  @sveltejs/kit: ^2.69.3 => 2.69.3 
  @sveltejs/vite-plugin-svelte: ^6.2.4 => 6.2.4 
  svelte: ^5.56.5 => 5.56.5 
  vite: ^7.3.6 => 7.3.6

Severity

serious, but I can work around it

(current hacky workaround may be to call myForm.fields.myField.set(...) inside of an $effect for a managed field)

Additional Information

Notably:

• new FormData(myForm.element) inside .enhance() shows all the expected values client-side
• in the server-side remote form function, the values are all as expected

Also, I could have sworn that this used to work fine in a previous sveltekit version, and thought perhaps v2.68.0 introduced a regression. But even after reverting way back to February (v2.52.2) this bug is present.