初回アクセスでBootstrapの変数が読み込まれない

サーバー立ち上げ後のアクセスでBootstrapの変数が読み込まれない

ポートフォリオ作成中に、発生しました。
Bootstrap4.2 を使用しており、
_variables.scssファイル内にデフォルトで入っている $info というシアン色の変数が、初回アクセスで
読み込まれません。

詳細

$ docker-compose upコマンド にてrailsサーバーを起動し、アプリにアクセスすると、上記画像のように、Bootstrapで定義している変数の読み込みに失敗します。

このエラー画面が表示されてから、何度かページ再読み込みを行うと、正常にアクセスできます。

考えていること

app/assets/layouts/application.html.hamlにて、Bootstrapのscssが読み込まれる前にhtmlを読み込んでいるため、
このエラーが出ていると考えています。
(※表示するhtmlには、class: 'bg-info' 等のBootstrapのクラスを使用しています。 )

試したこと

・= stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track': 'reload'
この行をapplication.html.hamlの<head>タグ内の一番上に配置

・= javascript_include_tag 'application', 'data-turbolinks-track': 'reload'
この行を<head>タグ内に移動(現在、Bodyタグ最下部に配置している)

・app/assets/stylesheets/application.scssにて、 *= require_selfを* require_selfに変更

コード

**app/views/layouts/application.html.haml**

!!!
%html
  %head
    %meta{ :content => 'text/html; charset=UTF-8', 'http-equiv' => 'Content-Type' }/
    %meta{ name: 'viewport', content: 'width: device-width, initial-scale: 1, minimum-scale: 1, user-scalable: yes' }
    %title= full_title(page_title: yield(:title))
    %link{ href: 'https://use.fontawesome.com/releases/v5.6.1/css/all.css', rel: 'stylesheet' }
    = csrf_meta_tags
    = csp_meta_tag
    = stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track': 'reload'
  %body
    = render 'layouts/header'
    .container-fluid.main-container-fluid.position-relative
      .flash-messages
        = render 'layouts/flash_messages'
      = yield
    = render 'layouts/footer'
    .p-5= debug(params) if Rails.env.development?
    = javascript_include_tag 'application', 'data-turbolinks-track': 'reload'

**app/assets/stylesheets/application.scss**

/*
 * This is a manifest file that'll be compiled into application.css, which will include all the files
 * listed below.
 *
 * Any CSS and SCSS file within this directory, lib/assets/stylesheets, or any plugin's
 * vendor/assets/stylesheets directory can be referenced here using a relative path.
 *
 * You're free to add application-wide styles to this file and they'll appear at the bottom of the
 * compiled file so the styles you add here take precedence over styles defined in any other CSS/SCSS
 * files in this directory. Styles in this file should be added after the last require_* statement.
 * It is generally better to create a new file per style scope.
 * require_self
 */
@import "bootstrap_custom";
@import "custom";
@import "responsive"

**app/assets/stylesheets/bootstrap_custom.scss**

/* 必須 */
@import "./scss/functions";
@import "./scss/variables";
@import "./scss/mixins";

/* 使用中コンポーネント */
@import "./scss/dropdown";
@import "./scss/forms";
@import "./scss/nav";
@import "./scss/navbar";
@import "./scss/card";
@import "./scss/alert";
@import "./scss/grid";
@import "./scss/utilities";
@import "./scss/reboot";
@import "./scss/transitions";
@import "./scss/type";
@import "./scss/list-group";
@import "./scss/buttons";
@import "./scss/badge";
@import "./scss/tables";
@import "./scss/modal";
@import "./scss/close";

**app/assets/javascripts/application.js**

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require rails-ujs
//= require activestorage
//= require turbolinks
//= require jquery3
//= require popper
//= require bootstrap
//= require_tree .

**app/assets/stylesheets/custom.scss**

@import "bootstrap_custom";
/* --共通変数-------------------------- */
$header-height: 56px;
/* --共通設定-------------------------- */
body{
  background-color: lightgray;
}
form {
  width: 100%;
}
.main-container-fluid {
  min-height: 100vh;
  margin-top: $header-height;
  padding: 0;
}
.main-container-fluid > .container {
  padding-top: 50px;
  padding-bottom: 50px;p
}
a {
  text-decoration: none !important;
  color: black;
  opacity: 1.0;
  &:hover {
    opacity: 0.8;
    color: $info; ------------------------------ここでエラー発生！
  }
}

現在のディレクトリ構造は下の画像のようになっています。

開発環境

**Gemfile**

ruby '2.6.6'

gem 'bootsnap', '>= 1.1.0', require: false
gem 'bootstrap', '~> 4.3.1'
gem 'bootstrap4-kaminari-views'
gem 'carrierwave'
gem 'coffee-rails', '~> 4.2'
gem 'counter_culture'
gem 'devise'
gem 'devise-i18n'
gem 'dotenv-rails'
gem 'haml_lint'
gem 'haml-rails'
gem 'jbuilder', '~> 2.5'
gem 'jquery-rails'
gem 'kaminari'
gem 'mini_magick'
gem 'omniauth', '1.9.1'
gem 'omniauth-facebook'
gem 'omniauth-rails_csrf_protection'
gem 'puma', '~> 3.11'
gem 'rails', '~> 5.2.1'
gem 'rails-i18n', '~> 5.1'
gem 'sass-rails'
gem 'turbolinks', '~> 5'
gem 'uglifier', '>= 1.3.0'