Hello RWB.
There are simularities regarding encoding strings for JSON. It seems that the nesting is the issue in my case: the outer json is ok, but the inner data json is messed up.
12/26/2016 11:38:59 PM> Device: [3e0025000447343138333038], Data:[{"data":"{ "t":68.562981, "h":42.400879 }","device_id":"3e0025000447343138333038","event":"temperature","published_at":"2016-12-26T22:38:59.707Z"}]
This is what the IoTHub Device Explorer shows when I try:
char payload[256];
snprintf(payload, sizeof(payload), "{ "t":%f, "h":%f }", tempf, humidity);
Particle.publish("temperature", payload);
btw. I checked the logging and it shows sound JSON... This confirms the encoding towards the IoTHub needs some love.