Electron Overheating and Draining Battery

I have two electrons that heat up in a matter of seconds and drain battery at faster rate. This issue occurs using the battery provided in the package and this battery. Here is some very rough data collected on a single unit on startup. These data were collected after the battery was disconnected and the device was allowed to cool to room temperature. Temperature is measured by an IR thermometer. It takes ~1minute for the device to take measurements, publish data to cloud and sleep. There are no peripherals attached to the electron for this test.

Time “Temp(°C)” “BattSOC (%)” “Details”
8:52:00 24.1 77.1 Battery plugged in
8:52:30 31.8 Take measurements
8:53:05 33 Go to Sleep
8:54:05 38.09
8:58:05 44.5
9:00:00 45 76.49 Wake from Sleep

The electron continues to stay >45C for the remainder of battery life.
Sleep is configured as follows:
SystemSleepConfiguration config;

config.mode(SystemSleepMode::STOP)
    .network(NETWORK_INTERFACE_CELLULAR)
    .flag(SystemSleepFlag::WAIT_CLOUD)
    .gpio(RI_UC, RISING)
    .duration(sleepTime);

This only occurs in some units (currently 2 of 17 that I know of).

What causes this heating?
-Kara