Why does the wrong object get tested in this RSpec setup?

I'm having trouble with an RSpec test, and I suspect it may be to do with my explicit use of subject in the before block. Notably, I am testing ActiveRecord objects that have a has_many / belongs_to relationship. This is my failing test:

subject { FactoryGirl.create(:parent) }
let(:child) { FactoryGirl.build(:child) }

context "with added child object" do
  before { subject.children << child }
  its(:foo) { is.expected_to eq("bar")
end

In my parent model I have some simple logic based on adding the child record which works outside of the test. Since it doesn't work in the test, I switched to writing the spec part out long-form to try and understand why:

before do
  puts "subject is #{subject}"
  puts "child is #{child}"
  subject.children << child 
  puts "#{child_to_attach} is now attached to #{child_to_attach.parent}"
end

it "has the correct response" do
  puts "testing against subject #{subject}"
  expect(subject.foo).to eq("bar")
end

And the output I got suggests that something weird is happening - that the subject I attach the child to is a different one than the rest of the test:

subject is #<Parent:0x00561eddf1a7a0>                                                  
child is #<Child:0x00561edcdd7fb0>
#<Child:0x00561edcdd7fb0> is now attached to #<Parent:0x00561edd11c040>
testing against subject #<Parent:0x00561eddf1a7a0>

Am I doing something wrong with subject to cause this behaviour? Is there a better way to write this test?