`Not found: /__data.json` when use form actions with `use:enhance`

Describe the bug

I have this code:

<!-- routes/+page.svelte -->

<script>
	import { enhance } from '$app/forms';
</script>

<form method="POST" use:enhance>
	<input type="text" name="text" id="textid" />
	<button type="submit">submit</button>
</form>

// routes/+page.server.js

export const actions = {
	default: () => {
		return {
			success: true
		};
	}
};

when I submit the form, the following error occurs:

Not found: /__data.json
Error: Not found: /__data.json
    at resolve (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/runtime/server/index.js:264:13)
    at resolve (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/runtime/server/index.js:295:5)
    at Object.handle (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:343:66)
    at respond (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/runtime/server/index.js:292:40)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:413:22

exactly the same happens if they are named actions, when using use:enhance I get the same error. I thought it was related to #7410, but I updated to the latest version and the error described still occurs

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-kbaf32?file=src/routes/+page.svelte

other option (local)

npm create svelte@latest 
// zero configuration
add files /+page.svelte and /+page.server.js

Logs

Not found: /__data.json
Error: Not found: /__data.json
    at resolve (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/runtime/server/index.js:264:13)
    at resolve (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/runtime/server/index.js:295:5)
    at Object.handle (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:343:66)
    at respond (file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/runtime/server/index.js:292:40)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async file:///C:/Users/dange/Documents/dev/sveltekit/node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:413:22

System Info

System:
    OS: Windows 10 10.0.22621
    CPU: (16) x64 AMD Ryzen 9 5900HS with Radeon Graphics
    Memory: 8.55 GB / 15.41 GB
  Binaries:
    Node: 18.0.0 - C:\Program Files\nodejs\node.EXE
    npm: 8.19.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.22621.755.0), Chromium (107.0.1418.24), ChromiumDev ((@(&"C:/Users/dange/AppData/Local/Programs/oh-my-posh/bin/oh-my-posh.exe" init pwsh --config="C:\Users\dange\AppData\Local\Programs\oh-my-posh\themes\jandedobbeleer.omp.json" --print) -join "`n") | Invoke-Expression)
    Internet Explorer: 11.0.22621.1
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.85
    @sveltejs/kit: next => 1.0.0-next.528
    svelte: ^3.44.0 => 3.52.0
    vite: ^3.1.0 => 3.2.1

Severity

blocking all usage of SvelteKit

Additional Information

I can do a minimal reproduction, but I literally just start a new project and try to use form actions with use:enhance

[KarolusD]

Same here, reverting back to: “@sveltejs/kit”: “1.0.0-next.527” helps.

[boian-ivanov]

I’ve just about wanted to report this issue. I’ve been getting it on the root route +page.server.js/ts file, but I’ve been able to also replicate it with the default create-svelte app. Stackblitz link here. In my experience if you have a +page.server.js/ts in the root of the routes, the /__data.json won’t exist so it can’t fetch any of the data that you are trying to pass at prerender. On full page load it works, but on prerender it doesn’t. Seems like quite the braking issue.

Edit: This issue seems to only exists on server only routes. If you make the +page.server.js/ts to a +page.js/ts this doesn’t seem to happen. But obviously you may need some API data on root that may require secret API keys.

Edit 2: as a momentary fix, I’ve been able to downgrade to 527 and the issue doesn’t appear anymore. So I assume that it comes from the changes in this commit.