I wonder if that could be the culprit. In general, the application has 1 second to respond, or even 2 seconds for the second receive window. So, maybe the gateway is not expecting an immediate response, but is expecting you to send it to some incoming port?
So, that payload, the Base64-encoded word hi, is not encrypted. I wonder if that's really done on the fly for you, but you might be right, I don't know. Also, I assume you're sure the node EUI is correct?
Finally, using the same txmsgid message id in each response might be troublesome too? I know the nodes and servers are keeping track of counters, but I'm not sure if that's related to the message id.