Awesome Nested Set after_create Bug

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 'activerecord', '6.0.0.rc1'
  gem 'sqlite3', '1.4.1'
  gem 'awesome_nested_set', github: 'collectiveidea/awesome_nested_set', branch: 'master', require: false
end

require "active_record"
require "minitest/autorun"
require "logger"
require 'awesome_nested_set'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :categories do |t|
    t.string :name
    t.string :slug
    t.integer :parent_id, :null => true, :index => true
    t.integer :lft, :null => false, :index => true
    t.integer :rgt, :null => false, :index => true

    # optional fields
    t.integer :depth, :null => false, :default => 0
    t.integer :children_count, :null => false, :default => 0
  end
end

class Category < ActiveRecord::Base
  acts_as_nested_set

  # Won't work
  after_create :update_slug

  # Will work
  # after_create_commit :update_slug

  def update_slug
    self.update! slug: self.name.parameterize
  end
end

class BugTest < Minitest::Test
  def test_association_stuff
    # Create root categories
    maths = Category.create! name: "Maths"

    # Create Testing as root.
    class_1 = Category.create! name: "Class 1", parent_id: maths.id
    assert_equal 1, class_1.depth
    assert_equal 1, class_1.ancestors.count
  end
end