it seems to be fault coming from your power ic, power ic (PM) do the charging and discharging functioning in the logic board. it use few methods for charging the battery,
1)USB mode charging
2)AC mode charging
*)Normal charging (400-500 mA)
*)Rapid/fast charging(600<800 mA)
also it change the charging profile according to charged percentage %
0%-15% : Fast charging
15%-80% : Normal charging
80%-100% : cutoff charging
accounting to your sounds your PM still working for fast charging but it not fully functional.
i would like to offer two solution
1) analyze the battery if it is OK
2) check and change the PM IC : your PM ic should be PM8018