timestamp loading issue on Rails 6.1

pg_default_error.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', '= 6.1'
  # gem 'rails'
  gem 'pg'
  gem 'byebug'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
begin
  config = { adapter: 'postgresql',
             host: 'localhost',
             port: '15432',
             username: 'postgres',
             database: 'pg-default-bug' }
  ActiveRecord::Base.establish_connection(config.except(:database))
  begin
    ActiveRecord::Base.connection.drop_database(config[:database])
  rescue StandardError
    nil
  end
  ActiveRecord::Base.connection.create_database(config[:database])
end

ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :invoices, force: true do |t|
    t.timestamp :issued_at, null: false, default: -> { 'now()' }

    t.timestamps
  end
end

class Invoice < ActiveRecord::Base
end

class BugTest < Minitest::Test
  def test_save
    invoice = Invoice.create!
    assert invoice.persisted?

    puts "Ruby #{RUBY_VERSION}"
    puts "Rails: #{Rails::VERSION::STRING}"
    puts "issued_at: #{invoice.issued_at}"
    puts "pluck issued_at: #{Invoice.pluck(:issued_at)}"

    # invoice.reload solves this
    assert invoice.issued_at.present?
  end
end

__END__

Run with:
```
$ ruby pg_default_error.rb
```

Output on different versions:

## fails on Rails 6.1 (Ruby 3.0 or 3.3)

Ruby 3.0.6
Rails: 6.1.0
issued_at:
pluck issued_at: [2023-12-07 17:06:16.179438 UTC]
F

Failure:
BugTest#test_save [pg_default_error.rb:59]:
Expected false to be truthy.


----

Ruby 3.3.0
Rails: 6.1.0
issued_at:
pluck issued_at: [2023-12-07 17:07:34.287919 UTC]
F

Failure:
BugTest#test_save [pg_default_error.rb:59]:
Expected false to be truthy.

----

Ruby 3.3.0
Rails: 6.1.0
issued_at:
pluck issued_at: [2023-12-07 17:08:54.936266 UTC]
F

Failure:
BugTest#test_save [pg_default_error.rb:59]:
Expected false to be truthy.
----

## works on Rails 7.1.2 (Ruby 3.0 or 3.3)

Ruby 3.3.0
Rails: 7.1.2
issued_at: 2023-12-07 17:08:15 UTC
pluck issued_at: [2023-12-07 17:08:15.154946 UTC]
.

----

Ruby 3.0.6
Rails: 7.1.2
issued_at: 2023-12-07 17:18:04 UTC
pluck issued_at: [2023-12-07 17:18:04.049521 UTC]
.

related to rails/rails#34237