RC Charger ToolkitRC M8 - calibration

[Sprinkler]

Do you guys have any idea what could cause such problems (voltage reading)?
I don't use it to charge batteries because it's either not possible to calibrate (as seen on the video), or once calibrated (eg. 1s voltage reading - it shows fairly stable readings in calibration menu) it can start to read wrong voltages during charging - especially with higher currents (it's not accurate).
Maybe it has something to do with resistors (voltage drop) and them reacting to temperature variation?
Overall the voltage reading is inaccurate and fluctuates.
It's been this way since the first day.
Video: https://streamable.com/l557l