Constantly discharging through non-existent USB connection

I’m not familiar with the s8. But most phones have a charging ic chip(with iPhone for example it’s called Tigris and a communicating chip called tristar). Sometimes a charging ic can be damaged,or go bad. This can cause problems. Have you tried factory resetting it(back up data first)?

Did you use a crappy charger and cable with it recently? Does the phone get warmer than normal? I know that mark Schaefer with iPad rehab works on Samsung phones on a microsoldering level. You could always contact them if you can’t figure it out. But if you’ve tried factory resetting it to run out likely software issues,then it’s got to be hardware related. If it’s not the charger port,it’s probably the charging circuit.