Bachi
Are you sure it's not the battery being on its end of life?
Such behavior of jumping battery percentage after heavy load on the processor (booting up the system is heavy load) is seen independent from the OS on all hardware.
It seems going back to normal when plugged in because modern chargers provide so much power that the processor is running on the charger input. And then there is plenty of power left to recharge at the same time.
A healthy battery will already loose 1-2 percent at booting, so if there are larger drops, there seems to be a problem with the battery who can't provide the needed power. In idle it can give out the needed wattage, once load sets in everything drops down.