We're seeing some strange behaviour after migrating from 3.1.0 and 4.0.2 (with 4.0.0 also exhibiting the same problems)

From time to time, RSpec tests will fail when #last fails to pick up the last record. Debugging through the tests shows that the record is there on the database, but #last is just not picking it up e.g.

> Job.all.pluck(:id)
[BSON::ObjectId('5566010f6865721d58d90200'), BSON::ObjectId('556601106865721d58f60200'), BSON::ObjectId('556601106865721d58130300')]
> Job.last.id
BSON::ObjectId('556601106865721d58f60200')

The behaviour is random, in that it will only happen about a third of the time when testing locally - even when the same seed is used for RSpec. Running on CircleCI seems to fail a lot more consistently though.

We're using the default Mongoid options, since consistency: :strong and safe: true have been deprecated, and it's just the single DB.

The failing tests are quite straightforward, and take the form:

let!(:job) {
  Fabricate(
    :job,
    job_items: [ @job_item = Fabricate(:job_item) ]
  )
}

before do
  @job_item.start_time = new_beginning
  @job_item.save  # this should update the parent job
end

it "denormalizes the description from job_item" do
  expect(Job.last.description).to include @job_item.ref
end

Tests aside (which we can definitely improve, and we can fix this particular case by using job.reload in lieu of Job.last), any ideas what might be happening here?

It's only affecting a small percentage of our tests, but I'm keen to understand whether this is a problem with our test suite, or with the application as a whole...