output.txt

$ ansible-playbook test.yml
PLAY [localhost] ***************************************************************

TASK [debug] *******************************************************************
ok: [localhost] => {
    "hosts": {
        "correct": "{\"host\":\"127.0.0.1\"}",
        "incorrect": {
            "host": "127.0.0.1"
        }
    }
}

PLAY RECAP *********************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0

test.yml

---
- hosts: localhost
  gather_facts: false
  vars:
    localhost: 127.0.0.1
    hosts:
      correct: '{"host":"127.0.0.1"}'
      incorrect: '{"host":"{{ localhost }}"}'
  tasks:
    - debug: var=hosts