MKR Fox 1200 doesn't wake up from sleep
Looks like same problem as described in https://forum.arduino.cc/index.php?topic=483636.0
Solution described in that thread:
Add

LowPower.attachInterruptWakeup(RTC_ALARM_WAKEUP, alarmEvent0, CHANGE);


to setup() and create a dummy function

void alarmEvent0() {
}