Script for Rails issue #39647 that replicates the reported issue.

issue_39647_bug.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails"
  gem "sqlite3"
end

require "active_record/railtie"
require "active_storage/engine"
require "tmpdir"

class TestApp < Rails::Application
  config.root = __dir__
  config.hosts << "example.org"
  config.eager_load = false
  config.session_store :cookie_store, key: "cookie_store_key"
  secrets.secret_key_base = "secret_key_base"

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  config.active_storage.service = :local
  config.active_storage.service_configurations = {
    local: {
      root: Dir.tmpdir,
      service: "Disk"
    }
  }
end

ENV["DATABASE_URL"] = "sqlite3::memory:"

Rails.application.initialize!

require ActiveStorage::Engine.root.join("db/migrate/20170806125915_create_active_storage_tables.rb").to_s

ActiveRecord::Schema.define do
  CreateActiveStorageTables.new.change

  create_table :tournament_registrations, force: true do |t|
    t.datetime :deleted_at
  end
end

class TournamentRegistration < ActiveRecord::Base
  has_one_attached :attachment

  default_scope { where(deleted_at: nil) }
end

require "minitest/autorun"

class BugTest < Minitest::Test
  def test_destroy_orphaned_by_default_scope_attachments
    registration = create_registration
    registration.update(deleted_at: Time.zone.now)

    ActiveStorage::Attachment.where(record_type: "TournamentRegistration").map(&:destroy)

    assert_equal 0, ActiveStorage::Attachment.count
  end

  def test_purge_orphaned_by_default_scope_attachments
    registration = create_registration
    registration.update(deleted_at: Time.zone.now)

    ActiveStorage::Attachment.where(record_type: "TournamentRegistration").map(&:purge)

    assert_equal 0, ActiveStorage::Attachment.count
  end

  def test_destroy_orphaned_by_delete_all_attachments
    create_registration
    TournamentRegistration.delete_all

    ActiveStorage::Attachment.where(record_type: "TournamentRegistration").map(&:destroy)

    assert_equal 0, ActiveStorage::Attachment.count
  end

  def test_purge_orphaned_by_delete_all_attachments
    create_registration
    TournamentRegistration.delete_all

    ActiveStorage::Attachment.where(record_type: "TournamentRegistration").map(&:purge)

    assert_equal 0, ActiveStorage::Attachment.count
  end
end

# Convenience method for creating a TournamentRegistration record.
#
# @return {TournamentRegistration} - Dummy record
def create_registration
  TournamentRegistration.create!(
    attachment: {
      content_type: "text/plain",
      filename: "dummy.txt",
      io: ::StringIO.new("dummy"),
    }
  )
end