Copying Content to the ResponseHandler methods is a TODO, so Content is always null. The HttpResponseMessage given to these methods is only a copy of the actual message instance.
.UseResponseHandler(new ResponseHandlerOptions{
ResponseReceived = response => YourAsyncMethod(response),
ResourceCreated = response => YourAsyncMethod(response),
ResourceUpdated = response => YourAsyncMethod(response),
ResourceRetrieved = response => YourAsyncMethod(response),
ResourceDeleted = response => YourAsyncMethod(response),
})