Accepts is not doing any parsing. It does content negotiation for the format the response will be formatted in. That’s completely indenpendent from the content on the request.
Parsing happens in Plug.Parsers in the endpoint. Plug.Parsers will however only run if body_params are still unfetched, so the error here is specific to manually reading the body here. And endpoint and router pipelines also need to be opted into being executed in phoenix tests with Phoenix.ConnTest.bypass_through/1.
So lots of things, which make having a proper conn.body value unnecessary in places.
