404 renders __NEXT_DATA__ as string to DOM when using RSC after deploy, locally no response

[KlotzJesse]

What version of Next.js are you using?

12.0.8-canary.5 up to 12.0.8-canary.7

What version of Node.js are you using?

local: 14.8.1 // vercel: 14

What browser are you using?

Chrome

What operating system are you using?

macOS

How are you deploying your application?

Vercel

Describe the Bug

When updating to newest canaries, starting by 12.0.8-canary.5, using serverComponents: true and concurrentFeatures: true the 404 page (default and custom) won't respond without error to the console.
When being deployed, the 404 renders, but with the NEXT_DATA json rendered as string to the dom.

When deployed, it's looking like:

on reproducible url: https://next-rsc-demo-e3pu88wfg-klotzjesse.vercel.app/thisdoesnotexist

and console:

Expected Behavior

Render in development.

And after deployment, without having NEXT_DATA as string in the dom.

To Reproduce

Update to canary 12.0.8-canary.5 up to .7
Activate in next.config.js

module.exports = {
  experimental: {
    reactRoot: true,
    concurrentFeatures: true,
    serverComponents: true,
  },
}

Deploy to Vercel / run yarn dev
Run into failure

Minimal Reproduction:
Failure (server): https://next-rsc-demo-e3pu88wfg-klotzjesse.vercel.app/rsc
Running (client): https://next-rsc-demo-e3pu88wfg-klotzjesse.vercel.app

Repo: https://github.com/KlotzJesse/next-rsc-demo/tree/rsc-fails-render-with-client-error

[KlotzJesse]

This render issues could somehow be related to #32514, i think? vice-versa

[KlotzJesse]

Still occuring on v12.0.8-canary.10:

Vercel reproduction deployment with v12.0.8-canary.10:
https://next-rsc-demo-klotzjesse.vercel.app/asd

I investigated:
Default error page won't render in dev (loading loop), but on production.

Custom 404 will render in dev env, showing a different String.

So I also found out some new errors in the local, dev env browser console:

After deployment it will show the same string as in the reproduction.

[balazsorban44]

This issue has been automatically locked due to no recent activity. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.