Issue with Ansible Check and Diff Mode

I was going thru this post (https://blog.networktocode.com/post/generating-diff-with-ansible/) where its mentioned that we can use check and diff mode together. I am trying to use that for gold config audit (audit pre decided config against what is present on device) for Cisco and Extreme devices. Below is my playbook –

# task to audit & heal ntp configuration
- name: Set NTP template
  ios_config:
     src: "{{domain}}_{{ansible_network_os}}_ntp.cfg"
     match: line
  register: ntp_result

Below is my template –

ntp server 10.19.70.1

Ansible highlighting that it is going to change the config. Below is what result I am getting. Surprisingly I am not getting any config diff.

"ntp_result": {
        "ansible_facts": {
            "discovered_interpreter_python": "/usr/bin/python"
        },
        "changed": true,
        "commands": [
            "configure ntp server add x.x.x.x"
        ],
        "failed": false,
        "updates": [
            "configure ntp server add x.x.x.x"
        ],
        "warnings": [
            "unable to perform diff against running-config due to check mode"
        ]
    }

Why I am not getting config diff? Any idea what is wrong?

        if module.params['diff_against'] == 'running':
            if module.check_mode:
                module.warn("unable to perform diff against running-config due to check mode")