unexpected behavior of #write_inheritable_attribute with Mongoid::Document included

mongoid_unexpected_behavior.rb

require 'mongoid'

class A
	include Mongoid::Document
	write_inheritable_attribute :attr_name, 'attr for A'
	class_inheritable_reader :attr_name
end

class B < A; end

puts A.attr_name.inspect, B.attr_name.inspect

class A1
	write_inheritable_attribute :attr_name, 'attr for A1'
	class_inheritable_reader :attr_name
end

class B1 < A1; end

puts A1.attr_name.inspect, B1.attr_name.inspect