SCSS AutoPrefixer Doesn't output any prefixes.

[Jwiggiff]

Description

I am trying to setup SCSS AutoPrefixer in my Jekyll site. I believe I have everything installed correctly, and there are no errors logged.
I have this in my Gemfile:

gem "jekyll", "~> 4.1.1"

group :jekyll_plugins do
  gem "jekyll-seo-tag"
  gem "jekyll-assets"
end

gem "autoprefixer-rails"

And this is the relevant content in my _config.yml:

plugins:
  - jekyll-seo-tag
  - jekyll-assets

assets:
  autoprefixer:
    browsers:
    - "since 2012"

Versions:
bundle info jekyll:

  * jekyll (4.1.1)
        Summary: A simple, blog aware, static site generator.    
        Homepage: https://jekyllrb.com
        Path: D:/Ruby27-x64/lib/ruby/gems/2.7.0/gems/jekyll-4.1.1

bundle info jekyll-assets:

  * jekyll-assets (1.0.0)
        Summary: jekyll-assets-1.0.0
        Homepage: http://jekyll-assets.github.com/jekyll-assets
        Path: D:/Ruby27-x64/lib/ruby/gems/2.7.0/gems/jekyll-assets-1.0.0

Steps

• Add gem "jekyll-assets" to Gemfile
• Add - jekyll-assets to plugins section in _config.yml
• Add assets configuration in _config.yml

Output

No prefixed SCSS in _site directory

Expected

Prefrixed SCSS properties. (Specifically flex related prefixes)