you can try similar thing as this project Battery Powered ESP8266 Temperature/Humidity Monitor with DHT11 . it sleeps for 30 mins and when awake it reads temp and time stamp from RTC and sends to cayenne. then sleep again. this can solve two major problem. if you are running on battery it will not drain the battery power and when the device comes online it will send temp and timestamp thus solving your problem of offline and not receiving data.
yes this is a bug and i will report it to team.