As spiff72 also pointed out, negative weights are displayed as positive. That's quite confusing for our students. For now, I've addressed this by using the raw ADC data as a zero point after an offset (tare). If the value during a measurement is greater than the zero point, then the value of 'scales.getWeight()' is positive. If the value is smaller, then I multiply 'scales.getWeight()' by -1.
I hope that issue is solved in the next library update.