ActionMailer deliver_later works in browser but not in test suite - ruby-on-rails

Question

I'm in the process of porting most of my mailers in my Rails 5 app to use deliver_later instead of deliver_now.
One of my mailers works just fine when I test it in the browser, but not when I run it in an integration test. (I've even started my rails server using my test environment and tested in the browser using the exact same data as my integration test, and it still runs fine in the browser, but the integration test fails).
The integration test is failing with the error:
ERROR -- : Error performing ActionMailer::DeliveryJob (Job ID:
19b8c1c9-81e1-4209-b97b-0feb1e22fad3) from Async(mailers) in 65.64ms:
ActionView::Template::Error (undefined method `message_text' for
nil:NilClass):
The line in my email template triggering the error is:
<p><%= simple_format(#message.message_text )%></p>
I'm completely stumped figuring out why my #message object is nil when the mailer is called from an integration test but not in the browser.
Relevant code:
jobs_controller:
cj = ContactJob.create!(job_id: #job.id, contact_id: id, message:message)
...
cj.send_job_email
contact_job.rb:
class ContactJob < ApplicationRecord
belongs_to :message
belongs_to :contact
belongs_to :job
validates :contact, uniqueness: {scope: :job}
validates :message, presence:true
def send_job_email()
ContactJobMailer.job(self.id).deliver_later
end
end
contact_job_mailer.rb:
def job (contact_job_id)
#cj = ContactJob.find(contact_job_id)
#contact=#cj.contact
#job= #cj.job
#message= #cj.message
mail to: #contact.email, subject: 'test'
end
integration test:
patch update_contacts_user_job_path(#user, #job), params: {job: {contact_ids: [ #contact2.id], message: {message_text: "This is a test"}}}
It's only the #message object that is nil, and only when I run it as part of an integration test, everything works fine in the browser.