Low Power library messing up serial text
The problem is most likely that the system is going to sleep while it's still sending the serial data.

Forcing all the serial data to be sent before you go to sleep should fix the problem (serial data is sent in the background using an interrupt so as to keep sketch slowdown to a minimum):

Serial.flush();
LowPower.powerDown(SLEEP_1S, ADC_OFF, BOD_OFF);