SerialUSB cause memory leak after LowPower.sleep(), using ArduinoLowPower lib. samd21 cpu

Every wakeup causes some memory loss, eventually causing CPU reset. However if USB is not plugged in, no memory is lost.
using USBDevice.detach(); and USBDevice.attach();, open/close port does not help.
Using samd version 1.6.17
Test RAM memory usage via:
extern "C" char *sbrk(int i);
int FreeRam (void)
{
  char stack_dummy = 0;
  return &stack_dummy - sbrk(0);
}