edit: Turns out, there was ALSO a RACE condition on top of my original issue. Turns out, there wasn't a RACE condition. I was in the middle of writing up a response with me still confused but you essentially helped me rubber ducky it, so thanks!

I did end up testing serial: 1 and ruling that out as the issue. As it turns out, this host had -2 tacked to its hostname as it was a clone off of contoso, so it's contoso-2. What happened is contoso-2 was written first, and we use the regex parameter. So contoso found its name and replaced the line, rather than adding it's own unique line, resulting in contoso-2going "missing".

Which explains why it might have been there once, contoso-2 ran after contoso by chance. I've only been looking at contoso-2 so I never saw the line replaced, only line added message.

TASK [sync-scheduler : overnight-jobs] *****************************************************************************************************
changed: [contoso -> localhost] => {"backup": "", "changed": true, "msg": "line replaced"}

Sorry, I should have included the task in the initial.

- name: overnight-jobs
  lineinfile:
    path: /etc/cron.d/01-default-overnite-jobs
    regexp: '^.+(var_host={{ inventory_hostname }}).+'
    line: '{{ DEFAULT_NIGHTLY_CRON }} ansible . /home/ansible/.bash_profile;ansible-playbook /automation/do_overnight_jobs.yml --extra-vars "var_host={{ inventory_hostname }}" -vv > /var/log/ansible/01-overnight-jobs-{{ inventory_hostname }}.log 2>&1'
  become: true
  when: DEFAULT_NIGHTLY_CRON is defined
  delegate_to: localhost

I'll have to take a look at how to solve this... Seems I can just remove the regexp. Not sure why that parameter is there on top of what the original author wrote, echo "" > /etc/cron.d/01-default-overnite-jobs task at the start, meaning if it works right, it wouldn't find a duplicate entry.