I am seeing what I believe to be the same problem when communicating with the FX Luxor lighting controller.
In testing outside SmartThings hub, using socket/cosock fails, whereas using lua-http works just fine. The difference is that lua-http sends the request as single write (request, headers, body), whereas socket is sending it as 3 individual successive writes. I have no control over the code on the Luxor controller.
Using socket, I am using the standard socket.http.request. Is there a way to control it, such that a single write is issued? Other ideas? Am I missing something?