Relay multi-project configuration doesn't work

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/crimson-frog-go8s7s

To Reproduce

Provide a Relay configuration that makes use of the multi-project format:

module.exports = {
    root: ".",
    // For simplicity I'm defining just one project but it doesn't make any difference
    sources: {
      ".": "repro",
    },
    excludes: ["**/node_modules/**", "**/__mocks__/**", "**/__generated__/**"],
    projects: {
      "repro": {
        output: "__generated__",
        language: "typescript",
        schema: "schema.graphql",
      },
    },
  };

Start the app (both dev mode or build) and observe the Relay integration doesn't work.

Current vs. Expected behavior

⚠ Invalid next.config.mjs options detected: 
⚠     "compiler.relay.src" is missing, expected string
⚠ See more info here: https://nextjs.org/docs/messages/invalid-next-config

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4102
  Available CPU cores: 2
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 15.0.4-canary.23 // Latest available version is detected (15.0.4-canary.23).
  eslint-config-next: N/A
  react: 19.0.0-rc-380f5d67-20241113
  react-dom: 19.0.0-rc-380f5d67-20241113
  typescript: 5.3.3
Next.js Config:
  output: N/A
Done in 3.69s.

Which area(s) are affected? (Select all that apply)

Not sure

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local)

Additional context

No response

Start with the linked CodeSandbox and the provided multi-project Relay configuration, then reproduce the behavior in both next dev and next build. Trace how this configuration is read and why next.config.mjs reports compiler.relay.src as missing. Done means the multi-project configuration works in both stages without that warning.