Rails 6 Webpacker::Manifest::MissingEntryError only in CI - ruby-on-rails

Question

Rails 6.1
Ruby 2.7.2
Webpacker 5.4.3
All my tests pass when I run them locally. The app works locally and in the staging environment. But when the tests run in GitHub actions, I get this error:
1) RenderController #testimonial accepts requests with valid tokens
Failure/Error: = javascript_pack_tag 'survey'
ActionView::Template::Error:
Webpacker can't find survey.js in /home/runner/work/musashi/musashi/public/packs-test/manifest.json. Possible causes:
1. You want to set webpacker.yml value of compile to true for your environment
unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
Your manifest contains:
{
}
# ./app/views/layouts/raw_asset.html.haml:9:in `_app_views_layouts_raw_asset_html_haml___3092456843941673586_361500'
# ./app/controllers/render_controller.rb:10:in `testimonial'
# ./spec/requests/render_controller_spec.rb:36:in `block (3 levels) in <main>'
# ------------------
# --- Caused by: ---
# Webpacker::Manifest::MissingEntryError:
# Webpacker can't find survey.js in /home/runner/work/musashi/musashi/public/packs-test/manifest.json. Possible causes:
# 1. You want to set webpacker.yml value of compile to true for your environment
# unless you are using the `webpack -w` or the webpack-dev-server.
# 2. webpack has not yet re-run to reflect updates.
# 3. You have misconfigured Webpacker's config/webpacker.yml file.
# 4. Your webpack configuration is not creating a manifest.
# Your manifest contains:
# {
# }
# ./app/views/layouts/raw_asset.html.haml:9:in `_app_views_layouts_raw_asset_html_haml___3092456843941673586_361500'
Finished in 1 minute 55.52 seconds (files took 5.17 seconds to load)
106 examples, 1 failure
I can't reproduce it anywhere else. This blog post describes what sounds like the same error, but the solution did not work for me.
I found this webpacker issue, which seems to describe my exact problem. But the issue is closed and the most common solution is to upgrade to newer versions of webpack/webpacker which I'm already running.