Hidden templateKey fields not saving to .md file

# Bug report

**What is the current behavior?**
The hidden fields for new pages I am creating are not being saved to the .md file that Netlify CMS creates, so Gatsby fails to build, as there is no path to a component.

**If the current behavior is a bug, please provide the steps to reproduce.**
I created a new page in the config.yml file, adding the following under the file collection:

I then navigated to the /admin page, and edited the new Sidebar page. I saw a new file appear at /pages/sidebar/index.md. However, the hidden templateKey field was not present, and the compilation failed at this point.

I have pushed my repo to [this repository](https://github.com/joseph-facta/gatsby-netlify-test-two).

**What is the expected behavior?**
I would expect to see the hidden fields I placed in the config.yml file to appear in the final index.md file.

**Other relevant information:**

System:
OS: Windows 10 10.0.19043
CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
Binaries:
Node: 16.13.1 - C:\Program Files\nodejs\node.EXE
npm: 8.1.2 - C:\Program Files\nodejs\npm.CMD
Browsers:
Edge: Spartan (44.19041.1266.0), Chromium (101.0.1210.32)
npmPackages:
gatsby: ^4.12.1 => 4.12.1
gatsby-plugin-image: ^2.0.0 => 2.3.0
gatsby-plugin-netlify: ^4.2.0 => 4.2.0
gatsby-plugin-netlify-cms: ^6.12.1 => 6.12.1
gatsby-plugin-purgecss: ^6.0.0 => 6.1.0
gatsby-plugin-react-helmet: ^5.0.0 => 5.3.0
gatsby-plugin-sass: ^5.0.0 => 5.3.0
gatsby-plugin-sharp: ^4.0.0 => 4.3.0
gatsby-remark-copy-linked-files: ^5.0.0 => 5.3.0
gatsby-remark-images: ^6.0.0 => 6.3.0
gatsby-remark-relative-images: ^0.3.0 => 0.3.0
gatsby-source-filesystem: ^4.0.0 => 4.3.0
gatsby-transformer-remark: ^5.0.0 => 5.3.0
gatsby-transformer-sharp: ^4.0.0 => 4.3.0
npmGlobalPackages:
gatsby-cli: 4.13.0

Node.js version: 16.13.1
NPM/Yarn version: NPM v8.1.2
Operating System: Windows 10
Additional tools: