（Rails6+Webpack）Herokuへのデプロイが失敗します。「Precompiling assets failed」

実現したいこと

Herokuにデプロイする際のPrecompileでのエラーを直したい。

エラー内容

Ruby on Rails6でWebアプリケーションの開発を行っております。
ローカル環境では問題なく動いており、いざ本番環境へ上げようと思ったらコンパイルが通らずエラーが起きてしまいました。

herokuへのデプロイエラー
1-----> Building on the Heroku-20 stack
2-----> Using buildpacks:
3       1. heroku/nodejs
4       2. heroku/ruby
5-----> Node.js app detected
6       
7-----> Creating runtime environment
8       
9       NPM_CONFIG_LOGLEVEL=error
10       USE_YARN_CACHE=true
11       NODE_VERBOSE=false
12       NODE_ENV=production
13       NODE_MODULES_CACHE=true
14       
15-----> Installing binaries
16       engines.node (package.json):  unspecified
17       engines.npm (package.json):   unspecified (use default)
18       engines.yarn (package.json):  unspecified (use default)
19       
20       Resolving node version 14.x...
21       Downloading and installing node 14.18.3...
22       Using default npm version: 6.14.15
23       Resolving yarn version 1.22.x...
24       Downloading and installing yarn (1.22.17)
25       Installed yarn 1.22.17
26       
27-----> Installing dependencies
28       Installing node modules (yarn.lock)
29       yarn install v1.22.17
30       [1/4] Resolving packages...
31       [2/4] Fetching packages...
32       [3/4] Linking dependencies...
33       warning " > @webpack-cli/serve@1.6.0" has unmet peer dependency "webpack-cli@4.x.x".
34       warning " > webpack-dev-server@3.11.3" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
35       warning "webpack-dev-server > webpack-dev-middleware@3.7.3" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
36       [4/4] Building fresh packages...
37       Done in 20.87s.
38       
39-----> Build
40       
41-----> Pruning devDependencies
42       yarn install v1.22.17
43       [1/4] Resolving packages...
44       [2/4] Fetching packages...
45       [3/4] Linking dependencies...
46       warning " > @webpack-cli/serve@1.6.0" has unmet peer dependency "webpack-cli@4.x.x".
47       warning " > webpack-dev-server@3.11.3" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
48       warning "webpack-dev-server > webpack-dev-middleware@3.7.3" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
49       [4/4] Building fresh packages...
50       warning Ignored scripts due to flag.
51       Done in 3.31s.
52       
53-----> Caching build
54       - yarn cache
55       
56-----> Build succeeded!
57 !     This app may not specify any way to start a node process
58       https://devcenter.heroku.com/articles/nodejs-support#default-web-process-type
59 !     Unmet dependencies don't fail yarn install but may cause runtime issues
60       https://github.com/npm/npm/issues/7494
61-----> Ruby app detected
62-----> Installing bundler 2.2.33
63-----> Removing BUNDLED WITH version in the Gemfile.lock
64-----> Compiling Ruby/Rails
65-----> Using Ruby version: ruby-2.7.5
66###### WARNING:
67       Removing `Gemfile.lock` because it was generated on Windows.
68       Bundler will do a full resolve so native gems are handled properly.
69       This may result in unexpected gem versions being used in your app.
70       In rare occasions Bundler may not be able to resolve your dependencies at all.
71       
72       https://devcenter.heroku.com/articles/bundler-windows-gemfile
73-----> Installing dependencies using bundler 2.2.33
74       Running: BUNDLE_WITHOUT='development:test' BUNDLE_PATH=vendor/bundle BUNDLE_BIN=vendor/bundle/bin bundle install -j4
75       Fetching gem metadata from https://rubygems.org/.........
76   
77〜〜〜文字数のため省略〜〜〜
78 
79       Bundle completed (158.56s)
80       Cleaning up the bundler cache.
81-----> Detecting rake tasks
82-----> Preparing app for Rails asset pipeline
83       Running: rake assets:precompile
84       yarn install v1.22.17
85       [1/4] Resolving packages...
86       [2/4] Fetching packages...
87       [3/4] Linking dependencies...
88       warning " > @webpack-cli/serve@1.6.0" has unmet peer dependency "webpack-cli@4.x.x".
89       warning " > webpack-dev-server@3.11.3" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
90       warning "webpack-dev-server > webpack-dev-middleware@3.7.3" has unmet peer dependency "webpack@^4.0.0 || ^5.0.0".
91       [4/4] Building fresh packages...
92       Done in 20.14s.
93       I, [2022-01-28T09:02:56.525710 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/manifest-a72913f4c604bc8f97dd576fc8777bf029401f6af0e14b6a6c9d3874acfe73e4.js
94       I, [2022-01-28T09:02:56.526226 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/manifest-a72913f4c604bc8f97dd576fc8777bf029401f6af0e14b6a6c9d3874acfe73e4.js.gz
95       I, [2022-01-28T09:02:56.526428 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/default_background-6abe2f56e2518f18dffec30bbde601dfc0680f7d5c0b0021552522915fe18efd.jpg
96       I, [2022-01-28T09:02:56.529229 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/default_company-c5b59a9601ff87fadda0e0945b35db1dc63b738ec9ee4faf3a3a441b2d236c9c.jpg
97       I, [2022-01-28T09:02:56.529651 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/default_recruit-b34b9329ac986b0c58b748cec678856cb036baab593480d958bd9adc9f240d5b.png
98       I, [2022-01-28T09:02:56.530098 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/default_student-4f70deae037dae189a4f5634d500e533577ead60beab6f28ea6f10e6f255d8d1.png
99       I, [2022-01-28T09:02:56.531150 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/feature1-f67d07bd085801a4ae81cbd95eaa12788217a440478c55ecd1082bdd13ea0b00.png
100       I, [2022-01-28T09:02:56.531860 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/feature2-37a85410aea319012b78568625f85927890c646d2c04643ea24f1e93d8dab94c.png
101       I, [2022-01-28T09:02:56.532378 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/feature3-bafd842f2c350197dfb3adb7d694af0df68ace6e55e4be3a0171257d7676ef42.png
102       I, [2022-01-28T09:02:56.532905 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/logo-1-095be00800632a68e5df1658698e4e46cca06c19c1b83e6c7cc3f2366decd357.png
103       I, [2022-01-28T09:02:56.533354 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/logo-2-e38e33e25d766c0ece355629bfe055d314cae80f826fd76e6891d92111e63cf1.png
104       I, [2022-01-28T09:02:56.533892 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/sample_recruit-11dcdd47327d30a3676bc107a36fab671a989c6d7d4e5530b09efe43f4f0c37b.jpg
105       I, [2022-01-28T09:02:56.535372 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/top1-537ec8313eb304a59be7e0145caf762173c5ddb1aaa8d1eeb450ed890c47dd3e.png
106       I, [2022-01-28T09:02:56.536127 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/top2-afc098a7c158d490af8050c32c196f798a096e7b1012d1ce3e4557bc80d27064.png
107       I, [2022-01-28T09:02:56.536418 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/top3-129bb66bf34c23115fcd21b2443d69710119eeef77adaf61b1008d39e709b7f1.png
108       I, [2022-01-28T09:02:56.537047 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/application-627e93369abca9597620b54f850bd4468d1e9727b6a36476f3b614489e24fe81.css
109       I, [2022-01-28T09:02:56.537656 #3060]  INFO -- : Writing /tmp/build_c7dea3d9/public/assets/application-627e93369abca9597620b54f850bd4468d1e9727b6a36476f3b614489e24fe81.css.gz
110       Compiling…
111       Compilation failed:
112       
113 !
114 !     Precompiling assets failed.
115 !
116 !     Push rejected, failed to compile Ruby app.
117 !     Push failed

試したこと

・config.assets.compile = falseをtrueに変更（production.rb)
・プリコンパイル対象になっている空のファイルを削除（空のSCSSファイルがあったので削除しました）

・同じ環境でサンプルアプリを作り、Herokuにデプロイさせましたが成功しました。
（ログイン機能でFirebaseAuthenticationを使用しているのでそのせいですかね...）
・rails assets:precompile RAILS_ENV=developmentで開発環境でPrecompileしてみましたが、そちらは問題ありませんでした。
・Heroku buildpacksの設定・追加
heroku buildpacks:set heroku/ruby
heroku buildpacks:add --index 1 heroku/nodejs