pylon:
Have you tried publishing just the temperature value to the MQTT server instead of the lengthy JSON structure? Are you sure your MQTT server accepts messages of that length? Did you patch the PubSubClient library (it has a default maximum message length of 128)?
I think therein lies the issue. I thought it might be a hardware issue but its probably an issue with the library and its limitation. Do you by any chance know why the limitation is 128 bytes and if i may simply open the library files and change the value?