ActiveRecord's after_commit callback triggered once per row

0-test.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

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

  gem 'activerecord', "6.0.3"
  gem 'sqlite3'
end

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

Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :post_items, force: true do |t|
    t.string :title
  end
end

class PostItem < ActiveRecord::Base
  after_commit :my_callback, on: :update

  private
    def my_callback
      puts "#{id}: previous_changes = #{previous_changes.inspect}"
    end
end

class BugTest < Minitest::Test
  def test_stuff
    PostItem.create!
    PostItem.create!

    PostItem.transaction do
      PostItem.first.update!(title: "first post title update 1")
      PostItem.first.update!(title: "first post title update 2")
      PostItem.last.update!(title: "last post title update 1")
      PostItem.last.update!(title: "last post title update 2")
    end
  end
end

1-result.log

D, [2020-08-02T18:00:29.208603 #5622] DEBUG -- :    (0.1ms)  begin transaction
D, [2020-08-02T18:00:29.208798 #5622] DEBUG -- :   PostItem Create (0.1ms)  INSERT INTO "post_items" DEFAULT VALUES
D, [2020-08-02T18:00:29.209030 #5622] DEBUG -- :    (0.0ms)  commit transaction
D, [2020-08-02T18:00:29.209380 #5622] DEBUG -- :    (0.0ms)  begin transaction
D, [2020-08-02T18:00:29.209490 #5622] DEBUG -- :   PostItem Create (0.1ms)  INSERT INTO "post_items" DEFAULT VALUES
D, [2020-08-02T18:00:29.209598 #5622] DEBUG -- :    (0.0ms)  commit transaction
D, [2020-08-02T18:00:29.210048 #5622] DEBUG -- :    (0.0ms)  begin transaction
D, [2020-08-02T18:00:29.210218 #5622] DEBUG -- :   PostItem Load (0.1ms)  SELECT "post_items".* FROM "post_items" ORDER BY "post_items"."id" ASC LIMIT ?  [["LIMIT", 1]]
D, [2020-08-02T18:00:29.210661 #5622] DEBUG -- :   PostItem Update (0.0ms)  UPDATE "post_items" SET "title" = ? WHERE "post_items"."id" = ?  [["title", "first post title update 1"], ["id", 1]]
D, [2020-08-02T18:00:29.210884 #5622] DEBUG -- :   PostItem Load (0.0ms)  SELECT "post_items".* FROM "post_items" ORDER BY "post_items"."id" ASC LIMIT ?  [["LIMIT", 1]]
D, [2020-08-02T18:00:29.211132 #5622] DEBUG -- :   PostItem Update (0.0ms)  UPDATE "post_items" SET "title" = ? WHERE "post_items"."id" = ?  [["title", "first post title update 2"], ["id", 1]]
D, [2020-08-02T18:00:29.211340 #5622] DEBUG -- :   PostItem Load (0.0ms)  SELECT "post_items".* FROM "post_items" ORDER BY "post_items"."id" DESC LIMIT ?  [["LIMIT", 1]]
D, [2020-08-02T18:00:29.211581 #5622] DEBUG -- :   PostItem Update (0.0ms)  UPDATE "post_items" SET "title" = ? WHERE "post_items"."id" = ?  [["title", "last post title update 1"], ["id", 2]]
D, [2020-08-02T18:00:29.211746 #5622] DEBUG -- :   PostItem Load (0.0ms)  SELECT "post_items".* FROM "post_items" ORDER BY "post_items"."id" DESC LIMIT ?  [["LIMIT", 1]]
D, [2020-08-02T18:00:29.211973 #5622] DEBUG -- :   PostItem Update (0.0ms)  UPDATE "post_items" SET "title" = ? WHERE "post_items"."id" = ?  [["title", "last post title update 2"], ["id", 2]]
D, [2020-08-02T18:00:29.212094 #5622] DEBUG -- :    (0.0ms)  commit transaction
1: previous_changes = {"title"=>[nil, "first post title update 1"]}
2: previous_changes = {"title"=>[nil, "last post title update 1"]}