[DIGITALOCEAN]
localhost ansible_connection=local

[PROVISION]
TEST1
TEST2
TEST3

[DROPLETS]
TEST1 ansible_ssh_host=x.x.x.x
TEST2 ansible_ssh_host=y.y.y.y
See, this one never seems to make it in... It changes too, and they get unordered too.

---
- hosts: PROVISION
  gather_facts: no

  vars:
    do_token: TOKEN

  tasks:
  - name: Create new droplet
    digital_ocean: >
      state=present
      command=droplet
      name={{ inventory_hostname }}
      unique_name=yes
      size_id=512mb
      region_id=nyc3
      image_id=ubuntu-18-04-x64
      ssh_key_ids=KEY
      api_token={{ do_token }}
      wait=yes
      wait_timeout=500
    register: hostname
    delegate_to: localhost

  - name: Add host to Ansible config
    lineinfile:
      dest: "/etc/ansible/hosts"
      insertafter: '^[DROPLETS]'
      state: present
      line: "{{ hostname.droplet.name }} ansible_ssh_host={{ hostname.droplet.ip_address }}"
    delegate_to: localhost