Charging Issue

I recently bought my iPhone 12 mini from from a used market with a Battery Health of 87% and when I try to charge it, it charges only till 99% and it stops when I try plugging out the charger and charge it again it says the same “99% charged” I thought maybe the charger cable is the problem so I charge it with another cable which comes with 14 pro max and it does the same. Next day I charged it again and same thing happened but decreases to 97% from 99% but when I restart the phone it shows 100% for few secs and does down to 99% and then 98% in couple of secs ik that 12 mini drain’s battery quite fast but not charging till 100% is weird I tried turning off optimise charging and plug it again it does the same. Please help me solve this problem I’ve never experienced this problem in my iPad nor the 14 pro max