You need to use the same react and react-dom version for your packages/apps in your monorepo. Also put ^ before the version number.

For this specific case change your apps' A and B package.json dependencies to this:

    "react": "^19.1.0",
    "react-dom": "^19.1.0"

Source: There's a comment in Vercel's next repo that explains it in more detail: link