If you run ansible-playbook with -v you'll see this message
"dest": "/tmp",
"msg": "file already exists",
So the destination /tmp do exist and the ntp.conf is not created.
If you set the "dest: /tmp/" (trailing slash) and you run
ansible-playbook with -v you get this
"dest": "/tmp/ntp.conf"
and it work as you expected.
If it's a feature or a bug I don't know, I guess it's a feature.
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Kai Stian Olstad