Disappeared parent in child after_destroy

disappeared_parent.rb

class Parent
  include Mongoid::Document
  embeds_many :children
  before_create :create_child
  
  private
  def create_child
    children.build
  end
end

class Child
  include Mongoid::Document
  embedded_in :parent
  after_destroy :do_something_related_to_parent
  
  private
  def do_something_related_to_parent
    parent.children # ERROR: undefined method `children' for nil:NilClass
  end
end

Parent.create.children.destroy_all # no problem?!
Parent.create.children.first.destroy # error!