Unable to hide a singleton method created by an extend

If I create a module and extend it in an object, creating singleton_methods, I should be able to hide one of the object's singleton methods by making it private. This works in MRI Ruby 1.8.7, Ruby 1.9.1, and many of the derivatives (I've also tried it in MacRuby 0.8, and REE 1.8.7).

The following code behaves differently in MRI and JRuby 1.5.6

1. simple module
   module Xmod
   def amethod
   end
   end

1. create an object with singleton methods by extending Xmod
   x = Object.new
   x.extend Xmod
   p x.singleton_methods # ["amethod"] # we're all good here

1. change the singleton method to private
   class << x
   private :amethod
   end
2. shouldn't see private method any more, but JRuby still shows it
   p x.singleton_methods # MRI => [], JRuby 1.5.6 => ["amethod"]

Note, I tried to simplify the test by just trying:

y = Object.new
class << y
def another_method
end
private :another_method
end
p y.singleton_methods

But this test case works. The bug only shows up with a singleton method created by an extend.