Sounds like a board issue where the U2 Tristar Charging IC, a communication chip that controls charging of the battery and power being provided to the phone, has gone faulty from liquid damage.
It's going to cost quite a bit to get this problem fixed as it requires micro soldering to replace the chip which is usually on the other side of the board and is covered by metal shields.
You will have to look for someone that does micro-soldering if you want to get an issue like that fixed.
You could try charging port but that requires spending more money on trying out replacement parts.
Try to look for a shop with a no-fix no-fee model as they are generally good mobile phone repair shops.