Since it’s part of a plugin config you need -t all (this isn’t the default for backwards compatibility reasons) or -t callback:
$ ansible-config dump --only-changed -t callback
ara_default:
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api_client(env: ARA_API_CLIENT) = http
api_server(env: ARA_API_SERVER) = http://127.0.0.1:8082
localhost_as_hostname_format(/home/ec2-user/ansible-aws/ansible/ansible.cfg) = hostname
default:
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result_format(/home/ec2-user/ansible-aws/ansible/ansible.cfg) = yaml
minimal:
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result_format(/home/ec2-user/ansible-aws/ansible/ansible.cfg) = yaml