For some reason, tags aren't showing up on a taggable object when an tagger is specified.

testing the post

class Post < ActiveRecord::Base
  acts_as_taggable_on :tags
  belongs_to :user
end

>> p = Post.first
=> #<Post id: 1, ...>
>> p.is_taggable?
=> true
>> p.tag_list = "foo, bar"
=> "foo, bar"
>> p.save
=> true
>> p.tags
=> [#<Tag id: 1, name: "foo">, #<Tag id: 2, name: "bar">]

testing the user

class User < ActiveRecord::Base
  acts_as_tagger
  has_many :posts
end

>> u = User.first
=> #<User id: 1, ...>
>> u.is_tagger?
=> true
>> u.tag(p, :with => "hello, world", :on => :tags)
=> true
>> u.owned_tags
=> [#<Tag id: 3, name: "hello">, #<Tag id: 4, name: "world">]

refresh the post

>> p = Post.first
=> #<Post id: 1 ...>
>> p.tags
=> [#<Tag id: 2, name: "bar">, #<Tag id: 1, name: "foo">]

Where's the hello and world tags? Miraculously, if I modify the database directly to set tagger_id and tagger_type to NULL, the two missing tags will show up. I suspect there's something wrong with my User model? What gives?