`postcssNode: AtRule`: failed build

Unsure if this is a jekyll-postcss error due to the cryptic messages I get from Jekyll but will have a go here.

Issue

Build stalls with bundle exec jekyll s -l --trace

No error is thrown; the build just stalls after Jekyll starts and throws the following code:

Configuration file: /Users/gschiavon/Documents/dev/sites.nosync/xxx/_config.yml
PostCSS Server listening on port 8124...
            Source: /Users/gschiavon/Documents/dev/sites.nosync/xxx
       Destination: /Users/gschiavon/Documents/dev/sites.nosync/xxx/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
       Jekyll Feed: Generating feed for posts
         AutoPages: Disabled/Not configured in site.config.
        Pagination: Complete, processed 9 pagination page(s)
              WebP: Processing /Users/gschiavon/Documents/dev/sites.nosync/xxxy/assets/img
              WebP: Generator Complete: 0 file(s) generated
[...]
  postcssNode: AtRule {
    raws: { before: '\n  ', between: '', afterName: ' ' },
    type: 'atrule',
    name: 'apply',
    parent: Rule {
      raws: [Object],
      type: 'rule',
      nodes: [Array],
      parent: [Root],
      source: [Object],
      selector: '.form-control',
      lastEach: 14,
      indexes: [Object],
      [Symbol(isClean)]: false,
      [Symbol(my)]: true
    },
    source: { start: [Object], input: [Input], end: [Object] },
    params: '.py-3',
    [Symbol(isClean)]: false,
    [Symbol(my)]: true
  },
  plugin: undefined
}

Upon interrupting the process, I get:

^CTraceback (most recent call last):
        44: from /Users/gschiavon/.rubies/ruby-2.7.3/bin/bundle:23:in `<main>'
        43: from /Users/gschiavon/.rubies/ruby-2.7.3/bin/bundle:23:in `load'
        42: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/gems/2.7.0/gems/bundler-2.2.30/exe/bundle:37:in `<top (required)>'
        41: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/friendly_errors.rb:103:in `with_friendly_errors'
        40: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/gems/2.7.0/gems/bundler-2.2.30/exe/bundle:49:in `block in <top (required)>'
        39: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:25:in `start'
        38: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
        37: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:31:in `dispatch'
        36: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
        35: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
        34: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
        33: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:478:in `exec'
        32: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:23:in `run'
        31: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:58:in `kernel_load'
        30: from /Users/gschiavon/.rubies/ruby-2.7.3/lib/ruby/site_ruby/2.7.0/bundler/cli/exec.rb:58:in `load'
        29: from /Users/gschiavon/.gem/ruby/2.7.3/bin/jekyll:25:in `<top (required)>'
        28: from /Users/gschiavon/.gem/ruby/2.7.3/bin/jekyll:25:in `load'
        27: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/exe/jekyll:15:in `<top (required)>'
        26: from /Users/gschiavon/.gem/ruby/2.7.3/gems/mercenary-0.4.0/lib/mercenary.rb:21:in `program'
        25: from /Users/gschiavon/.gem/ruby/2.7.3/gems/mercenary-0.4.0/lib/mercenary/program.rb:44:in `go'
        24: from /Users/gschiavon/.gem/ruby/2.7.3/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `execute'
        23: from /Users/gschiavon/.gem/ruby/2.7.3/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `each'
        22: from /Users/gschiavon/.gem/ruby/2.7.3/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `block in execute'
        21: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/commands/serve.rb:86:in `block (2 levels) in init_with_program'
        20: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/command.rb:91:in `process_with_graceful_fail'
        19: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/command.rb:91:in `each'
        18: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/command.rb:91:in `block in process_with_graceful_fail'
        17: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/commands/build.rb:36:in `process'
        16: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/commands/build.rb:65:in `build'
        15: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/command.rb:28:in `process_site'
        14: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/site.rb:80:in `process'
        13: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/site.rb:211:in `render'
        12: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/site.rb:538:in `render_pages'
        11: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/site.rb:538:in `each'
        10: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/site.rb:539:in `block in render_pages'
         9: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/site.rb:547:in `render_regenerated'
         8: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/renderer.rb:63:in `run'
         7: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/renderer.rb:84:in `render_document'
         6: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/renderer.rb:104:in `convert'
         5: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/renderer.rb:104:in `reduce'
         4: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/renderer.rb:104:in `each'
         3: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-4.2.1/lib/jekyll/renderer.rb:106:in `block in convert'
         2: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-postcss-0.5.0/lib/jekyll/converters/postcss.rb:42:in `convert'
         1: from /Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-postcss-0.5.0/lib/jekyll-postcss/socket.rb:28:in `read'
/Users/gschiavon/.gem/ruby/2.7.3/gems/jekyll-postcss-0.5.0/lib/jekyll-postcss/socket.rb:28:in `gets': Interrupt

package.json

"devDependencies": {
    "autoprefixer": "^10.3.6",
    "cssnano": "^5.0.8",
    "postcss": "^8.3.10",
    "postcss-cli": "^7.1.0",
    "postcss-import": "^14.0.1",
    "purgecss": "^4.0.3",
    "tailwindcss": "^2.1.1",
    "tailwindcss-spinner": "^1.0.0",
    "@tailwindcss/typography": "^0.4.1",
    "postcss-color-function": "^4.1.0",
    "postcss-simple-vars": "^5.0.2",
    "stickyfilljs": "^2.1.0"
  },

Gemfile

source "https://rubygems.org"

gem "jekyll", "~> 4.2"
gem 'kramdown-parser-gfm', '~> 1.1'

group :jekyll_plugins do
  gem "jekyll-feed"
  gem "jekyll-sitemap"
  gem "jekyll-seo-tag"
  gem "jekyll-postcss"
  gem "jekyll-purgecss"
  gem 'jekyll-fontawesome-svg'
  gem 'jekyll-redirect-from'
  gem 'jekyll-paginate-v2'
  gem 'jekyll-webp'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem "tzinfo-data", platforms: [:mingw, :mswin, :x64_mingw, :jruby]

# Semantic indexing
gem 'classifier-reborn'

# Performance-booster for watching directories on Windows
gem "wdm", "~> 0.1.0" if Gem.win_platform?

Start by reproducing with bundle exec jekyll s -l --trace, then compare the Gemfile and package.json versions. Inspect jekyll-postcss/socket.rb at the read call shown in the traceback and determine why the PostCSS build stalls on the @apply rule. Done means the reported site builds without hanging and the failure is covered by a reproducible check.