Mongoid 2.4.9

Since upgrading from 2.4 to 2.4.9 I've started seeing an issue where assigning via the relationship of a belongs_to when there are two pointed at the same class, overwrites the first one's ID

class User
has_many :created_posts, :class_name => "Post", :inverse_of => :creator, :validate => false
has_many :assigned_posts, :class_name => "Post", :inverse_of => :assignee, :validate => false
end

class Post
belongs_to :creator, :class_name => "User"
belongs_to :assignee, :class_name => "User"
end

ruby-1.9.2-head :004 > u1 = User.create
=> #<User _id: 4fac35f0803544465b000001, _type: "User", ...>
ruby-1.9.2-head :005 > u2 = User.create
=> #<User _id: 4fac35fe803544465b000002, _type: "User", ...>
ruby-1.9.2-head :006 > p = Post.new
=> #<Post _id: 4fac3602803544465b000003, _type: nil, ...>
ruby-1.9.2-head :007 > p.creator = u1
=> #<User _id: 4fac35f0803544465b000001, _type: "User", ...>
ruby-1.9.2-head :009 > p.assignee = u2
=> #<User _id: 4fac35fe803544465b000002, _type: "User", ...>
ruby-1.9.2-head :010 > p.creator_id
=> BSON::ObjectId('4fac35fe803544465b000002') // <= notice this is the incorrect id
ruby-1.9.2-head :011 > p.assignee_id
=> BSON::ObjectId('4fac35fe803544465b000002')
ruby-1.9.2-head :012 > p.creator.id
=> BSON::ObjectId('4fac35f0803544465b000001') // <= although this one is correct