You probably need to calibrate the battery gauge. I read through the axp228 manual and added this feature to my kernel for A06/CM4.
The manual did not state HOW to calibrate thought 
So I guess just:
- charge it up to 100%
- activate the calibrate function
- echo 1 | sudo tee /sys/class/power_supply/axp20x-battery/calibrate
- let it drain to poweroff
- charge it up again.