Bundle splitting per page with createPages API stopped working after updating to Gatsby 4.5.0

[davidhoneyb]

Preliminary Checks

• [X] This issue is not a duplicate. Before opening a new issue, please search existing issues: https://github.com/gatsbyjs/gatsby/issues
• [X] This issue is not a question, feature request, RFC, or anything other than a bug report directly related to Gatsby. Please post those things in GitHub Discussions: https://github.com/gatsbyjs/gatsby/discussions

Description

After updating Gatsby from 4.4.0 to 4.5.0, the bundle split per page functionality stopped working. We use the createPages API to create pages dynamically. We expected each page to have its own bundle, but now it appears that the entire app is bundled together when running gatsby build.

Reproduction Link

https://github.com/HoneyBook/gatsby-minimal-reproduction

Steps to Reproduce

1. Clone the minimal reproduction repository: https://github.com/HoneyBook/gatsby-minimal-reproduction
2. Install dependencies: npm install
3. Build the project: npm run build
4. Observe the bundle splitting behavior in the build output.

Expected Result

Each page should have its own bundle as it did in Gatsby 4.4.0.

Actual Result

The entire app is bundled together, and bundle splitting per page is not working in the build output.

Environment

System:
    OS: macOS 14.4.1
    CPU: (10) arm64 Apple M1 Max
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.9.0 - ~/Library/Caches/fnm_multishells/56387_1719134227025/bin/node
    Yarn: 1.22.19 - ~/.nvm/versions/node/v14.19.3/bin/yarn
    npm: 10.1.0 - ~/Library/Caches/fnm_multishells/56387_1719134227025/bin/npm
  Browsers:
    Firefox: 113.0.2
    Safari: 17.4.1
  npmPackages:
    gatsby: ^5.13.5 => 5.13.6 
    gatsby-link: ^5.13.1 => 5.13.1 
    gatsby-plugin-breakpoints: ^1.3.11 => 1.3.11 
    gatsby-plugin-eslint: ^4.0.4 => 4.0.4 
    gatsby-plugin-manifest: ^5.13.1 => 5.13.1 
    gatsby-plugin-offline: 5.10.1 => 5.10.1 
    gatsby-plugin-react-helmet: 5.10.0 => 5.10.0 
    gatsby-plugin-react-i18next: 1.2.2 => 1.2.2 
    gatsby-plugin-react-svg: ^3.3.0 => 3.3.0 
    gatsby-plugin-sass: 5.10.1 => 5.10.1 
    gatsby-plugin-sitemap: 5.10.1 => 5.10.1 
    gatsby-plugin-split-css: ^2.0.3 => 2.0.3 
    gatsby-plugin-webfonts: ^2.3.2 => 2.3.2 
    gatsby-plugin-webpack-bundle-analyser-v2: 1.1.26 => 1.1.26 
    gatsby-source-filesystem: ^5.13.1 => 5.13.1 
    gatsby-transformer-gitinfo: ^1.1.0 => 1.1.0

Config Flags

No response