I can't get the http output plugin to send as json instead of binary data

[thayes-amplify]

For some reason my http output is sending the body as binary data. Anyone run into this? I can't get it to just send json.

Here is my simplified conf:

<match heartbeats>
    @type http
    @log_level trace

    content_type json
    endpoint "#{ENV['URL']}"
    http_method post
    error_response_as_unrecoverable false

    <buffer>
        flush_interval 1s
    </buffer>
</match>

I've tried also setting json_array true. In addition, when I change @type to stdout just to see what it looks like it looks exactly the way I want it to be sent in the request. When I output the body in the logs of the service it's hitting (a Python service), it's outputtingb'{json string ommitted}' indicating it's binary data.

What am I doing wrong?