Type narrowing not working properly in single-fetch / plain object returns from loaders

[AndreSilva1993]

I'm using React Router as a...

framework

Reproduction

Previously when using the json() method we were able to type narrow the loader data properly.

If we just return plain objects, TypeScript is still marking the property as undefined even though it never is. It also contains the other property as possibly that should never be present given the type narrowing.

Tested this with @remix-run/node / @remix-run/react and react-router v7 and the issue is present in both.

https://github.com/AndreSilva1993/single-fetch-type-narrowing -> React Router v7
https://github.com/AndreSilva1993/json-vs-single-fetch-type-narrowing -> remix-run/node and remix-run/react with v3_singleFetch enabled.

System Info

System:
    OS: macOS 15.2
    CPU: (10) arm64 Apple M1 Pro
    Memory: 87.89 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.11.0 - ~/.nvm/versions/node/v22.11.0/bin/node
    Yarn: 4.6.0 - ~/.nvm/versions/node/v22.11.0/bin/yarn
    npm: 10.9.0 - ~/.nvm/versions/node/v22.11.0/bin/npm
  Browsers:
    Chrome: 131.0.6778.267
    Safari: 18.2
  npmPackages:
    @react-router/dev: ^7.1.3 => 7.1.3
    @react-router/node: ^7.1.3 => 7.1.3
    @react-router/serve: ^7.1.3 => 7.1.3
    react-router: ^7.1.3 => 7.1.3
    vite: ^5.4.11 => 5.4.14

Used Package Manager

npm

Expected Behavior

I would expect the type narrowing to work as expected.

Actual Behavior

The returned object ends up being an union of all possible properties stating that they could be undefined rendering type narrowing unusable.