Weird scope bug?

scope.rb

class WorkingClass
  def call
    hidden_method
  end

  private
  def self.hidden_method
    "Found me!"
  end

  def hidden_method
    self.class.hidden_method
  end
end

class FailingClass
  def call
    hidden_method
  end

  class << self
    private
    def hidden_method
      "Found me!"
    end
  end

  private
  def hidden_method
    self.class.hidden_method
  end
end

WorkingClass.new.call
#=> "Found me!"

FailingClass.new.call
#=> private method `hidden_method' called for FailingClass:Class (NoMethodError)