Trying to reproduce bug in FactoryBot

factory_bot_bug.rb

require 'active_record'
require 'factory_bot'
require 'pry'


ActiveRecord::Base.establish_connection(:adapter => 'sqlite3',
                                        :database => File.dirname(__FILE__) + "/db.sqlite3")

ActiveRecord::Schema.define do
  self.verbose = false

  create_table :users, :force => true do |t|
    t.string  :username
    t.timestamps
  end
end

class User < ActiveRecord::Base; end

FactoryBot.define do
  factory :user do
  end
end

username = 'foo'

user = FactoryBot.create(:user, username: username)

username.frozen? # => false (but is true in production code)