rubymotionバグ？　block_given?がブロックの中で評価すると値が変わる

a.rb

def callback_caller(&block)
  p block_given?
  callback_caller2 do
    p block_given?
    block.call if block_given?
  end
end

def callback_caller2(&block)
  block.call
end

callback_caller do
  p 'origin'
end

# $ ruby -v
# ruby 1.9.3p374 (2013-01-15 revision 38858) [x86_64-darwin12.2.1]
# $ /Library/RubyMotion/bin/ruby -v
# RubyMotion (ruby 1.9.2) [universal-darwin12.0, x86_64]
# $ ruby a.rb
# true
# true
# "origin"
# $ /Library/RubyMotion/bin/ruby a.rb
# true
# false

sample.rb

# -*- coding: utf-8 -*-
# a part of api client
  def refresh_access_token(refresh_token, &block)
    body = {
      grant_type: "refresh_token",
      client_id: CONSUMER_KEY,
      client_secret: CONSUMER_SECRET,
      refresh_token: refresh_token,
    }

    p is_block_given = block_given? # => true
    BW::HTTP.post(TOKEN_URL, payload: body) do |response|
      if json = BW::JSON.parse(response.body.to_s)
        @refresh_token = json['refresh_token']
        @access_token  = json['access_token']
      end
      p block_given? # => false
      block.call if is_block_given # will execute
      block.call if block_given?   # will don't execute
    end
  end

# another file
  def viewDidAppear(animated)
    App.shared.delegate.api_client.
      refresh_access_token(App::Persistence['refresh_token']) do
      fetch_tweets
      updateViewConstraints
    end
  end