This is due to the fact that the Sen55 is stopped between measurements. The external power supply corrects this, but consumption is of the order of 160 mAh...
One wonders why an internal 600 mAh battery is essential to the operation of the device, and why it's impossible to supply power via the Grove HY-2.0 connector.

@Andreu You can try running the M5Burner to reflash your M5 AirQ with a good code. This may correct your problem.

VOC measurements are not performed when AirQ is in battery mode !