Private `Module.delegate` behavior is wrong?

gems.rb

source 'https://rubygems.org'

gem 'activesupport', '~> 5.0'
gem 'minitest'

private_delegate_behavior.rb

#! /usr/bin/env ruby
require "bundler"; Bundler.require

require "active_support/all"
require "minitest/autorun"

class Base
  def initialize(obj)
    @obj = obj
  end
end

describe "public delegates" do
  before do
    kls = Class.new(Base) do
      delegate :foo, to: :@obj
    end

    @base = OpenStruct.new(foo: 42)
    @obj = kls.new(@base)
  end

  specify "#foo is public" do
    assert_respond_to @obj, :foo
    assert @obj.method(:foo)
    assert_equal @obj.public_send(:foo), 42
  end

  specify "#foo calls underlying object's method" do
    assert_equal @obj.foo, 42
    def @base.foo; 99; end
    assert_equal @obj.foo, 99
    assert_equal @obj.public_send(:foo), 99
  end
end

describe "prefixed public delegates" do
  before do
    kls = Class.new(Base) do
      delegate :foo, to: :@obj, prefix: :magic
    end

    @base = OpenStruct.new(foo: 42)
    @obj = kls.new(@base)
  end

  specify "there is no #foo" do
    refute_respond_to @obj, :foo
  end

  specify "#magic_foo is public" do
    assert_respond_to @obj, :magic_foo
    assert @obj.method(:magic_foo)
    assert_equal @obj.public_send(:magic_foo), 42
  end

  specify "#magic_foo calls underlying object's method" do
    assert_equal @obj.magic_foo, 42
    def @base.foo; 99; end
    assert_equal @obj.magic_foo, 99
    assert_equal @obj.public_send(:magic_foo), 99
  end
end

describe "private delegates" do
  before do
    kls = Class.new(Base) do
      private(*delegate(:foo, to: :@obj))
    end
    @base = OpenStruct.new(foo: 42)
    @obj = kls.new(@base)
  end

  specify "#foo is private" do
    refute_respond_to @obj, :foo # Because it's private
    assert @obj.respond_to?(:foo, true) # This is how you check private respond_to?
    assert @obj.method(:foo) # Can always fetch, even if private
    assert_raises(NameError) do
      @obj.public_send :foo
    end
    assert_raises(NameError) do
      @obj.foo
    end
  end

  specify "#foo calls underlying object's method" do
    assert_equal @obj.send(:foo), 42
    def @base.foo; 99; end
    assert_equal @obj.send(:foo), 99
  end
end

describe "prefixed private delegates" do
  before do
    kls = Class.new(Base) do
      private(*delegate(:foo, to: :@obj, prefix: :magic))
    end

    @base = OpenStruct.new(foo: 42)
    @obj = kls.new(@base)
  end

  specify "there is no #foo" do
    refute_respond_to @obj, :foo
    refute @obj.respond_to?(:foo, true)
    refute @obj.method(:foo)
  end

  specify "#magic_foo is private" do
    refute_respond_to @obj, :magic_foo
    assert @obj.respond_to?(:magic_foo, true)
    assert @obj.method(:magic_foo)
  end

  specify "#magic_foo calls underlying object's method" do
    assert_equal @obj.send(:magic_foo), 42
    def @base.foo; 99; end
    assert_equal @obj.send(:magic_foo), 99
  end
end

describe "prefixed private delegates' ACTUAL BEHAVIOR" do
  before do
    kls = Class.new(Base) do
      def foo
        55
      end

      private(*delegate(:foo, to: :@obj, prefix: :magic))
    end

    @base = OpenStruct.new(foo: 42)
    @obj = kls.new(@base)
  end

  specify "pre-existing #foo method is made private (by `private`)" do
    refute_respond_to @obj, :foo
    assert @obj.respond_to?(:foo, true)
    assert @obj.method(:foo)
  end

  specify "newly created #magic_foo is PUBLIC" do
    assert_respond_to @obj, :magic_foo
    assert @obj.method(:magic_foo)
  end

  specify "#magic_foo calls underlying object's method" do
    assert_equal @obj.send(:magic_foo), 42
    def @base.foo; 99; end

    assert_equal @obj.send(:magic_foo), 99
  end
end