Voltage spikes from lightning connecter

A few days ago I checked the voltage on a broken iPhone 6s at the battery connector on the main board when charging and it had like these weird spikes, and it started at 2V and then it immediately dropped to 0V and then reset, I tried to take the measurement both with and without a 1M ohm resistor but it gave me the same results. I checked a working iPhone 6 (as I don't have another 6s) and checked the voltages on that one too and it was more stable, which lead me to believe that the charging port was faulty. I have now changed the charging dock and I have the same problem. Any ideas of why I get these spikes or if it is normal?

Cheers