Phone turn on while charging
There is probably no such "feature". Maybe there was a power outage? Maybe it's a bug? If it happens again, contact Lenovo and send the phone in under warranty...

Btw, it wears down the battery quickly if you fully discharge it regularly. (https://lifehacker.com/5875162/how-often-should-i-charge-my-gadgets-battery-to-prolong-its-lifespan)