Adding local-only collections with _config_dev.yml or other methods

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TL;DR: When using two config files, any collection only defined in the first config file does not get created appropriately. How can I tell Jekyll to merge the lists of collections and front matter defaults in both config files, rather than overwrite them?

Background

I have a blog based on the Minimal Mistakes theme. It has several collections.

I have several normal ones (_posts, _pages, and _about). These ones should show up on the public website hosted by Cloudflare pages, and I want to see them when I’m hosing them locally with jekyll serve.

I have two collections that I only want to show up when I’m working and hosting locally - _docs and _examples. These host the Minimal Mistakes documentation and example posts, so I can access them locally and update them as I add new liquid snippets specific to my site. I do not want these to show up on the public site.

Because I need local configuration, I’ve done the standard thing, and I’m using two config files: _config.yml and _config_dev.yml.

To serve my site, I run bundle exec jekyll serve --config _config.yml,_config_dev.yml -h 10.0.0.165

_config.yml has the collection definition and front matter defaults for _posts, _pages, and_about. _config_dev.yml has the collection definition and front matter defaults for _docs and _examples. I’ve included those sections of the files below.

\_config.yml collections and front matter definitions 
### Collections

collections:
  # Posts has defaults.  Posts is the blog.
  # Pages has defaults
  about:
    output: true
    permalink: /:collection/:path/


### Post Front Matter defaults 
words_per_minute         : 200
defaults:
  # _posts
  - scope:
      path: ""
      type: posts
    values:
      layout: single
      author_profile: true
      read_time: true
      comments: true
      share: true
      related: true
      show_date: true
  # _pages
  - scope:
      path: "_pages"
      type: pages
    values:
      layout: single
      author_profile: true
  # _about
  - scope:
      path: "_about"
      type: about
    values:
      layout: single
      author_profile: false
\_config_dev.yml collections and front matter definitions 
### Additional Local Collections

collections:
  docs: #[[Local Config]]
    output: true
    permalink: /:collection/:path/
  examples: #[[Local Config]]
    output: true
    permalink: /:collection/:path/

### Local Config Post Front Matter defaults https://mmistakes.github.io/minimal-mistakes/docs/configuration/#front-matter-defaults

defaults:
  # _docs
  - scope:  #[[Local Config]]
      path: "_docs"
      type: docs
    values:
      layout: single
  # _examples
  - scope:  #[[Local Config]]
      path: "_examples"
      type: examples
    values:
      layout: single

Expected Behavior

When I serve my site locally, with bundle exec jekyll serve --config _config.yml,_config_dev.yml -h 10.0.0.165 Jekyll reads the _config.yml and adds the elements in collections and front matter defaults to an internal database.

Then it reads the _config_dev.yml file, and sees that the collections and defaults trees have different elements. It adds the new elements to its internal database. If any elements conflict, it replaces those values with the values from _config_dev.yml.

The Symptom(s)

When I serve my site locally, pages in the _about folder, the _posts folder, and the _pages folder render without any CSS styling.

Adding a layout to the front matter manually improves the appearance, but it does not match what appears on the public site.

Copying the collections and defaults sections from _config.yml to _config_dev.yml fixes the layout entirely.

My Hypothesis

When using both _config.yml and _config_dev.yml, Jekyll does not add the elements of the collections and defaults trees to an internal database, but rather sees that the trees are different between the two files, and overwrites the entire tree.

The Question

How can I do what I want to do?

How can I prevent some collections from appearing on the public-facing website, while still letting them appear when I serve the site locally?

Am I going to be stuck duplicating the collections and defaults sections, and being careful that any changes I make to the public collections’ configurations get propagated to both the _config.yml and _config_dev.yml files?

Or is there something clever I don’t know about that will let me automatically tell Jekyll “hey, I want you to merge these two lists, not replace them”

Do I need to use some method other than _config_dev.yml?

Thanks in advance!