Ok, just checked on this - turns out it's the USB interface chip takes a while to initialise when first powered on and raise the RX pin. Espruino boots up before it, checks the RX pin and decides nothing is there so it doesn't bother initialising the UART.
I think sending Break causes the USB UART to reboot Espruino - it then resets and sees the RX pin is high so then initialises the UART.
If you grab the latest cutting edge build now then it should work for you.
I also added support for the new accelerometer & magnetometer
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