HttpMessageContent serializes HTTP requests/responses inside a mulitpart/mixed body.

For requests, it sends the header Content-Type: application/http; msgtype=request.
For responses, it expects the header Content-Type: application/http; msgtype=response.

This is where msgtype=request is added in source:

This is where msgtype=response is validated in source:

HttpMessageContent should be configurable to not sent a msgtype parameter, and accept responses without a msgtype (given Content-Type: application/http).

As a common real-world example, the OData specification uses Content-Type: application/http without any further msgtype in both its batched requests and responses, see:
https://www.odata.org/documentation/odata-version-2-0/batch-processing/

Inside Microsoft, Dataverse exposes OData services and its batching endpoint is unusable with HttpMessageContent because:

• Dataverse throws when a batch has msgtype=request in its Content-Type;
• ReadAsHttpResponseMessageAsync() throws when parsing Dataverse response because it does not have msgtype=response.