Hello. I got Samsung Galaxy S8+ and using original stock charging adapter and cable. When i plug charging cable to device it shows fast charging and then after 2 seconds it sounds replug and shows cable charging. I noticed fast charging and cable charging full charge time is same. Searched google many people have same issue but no one finds a fix for it.
Well i had that problem too when i got my s8+. At first it was in the G955FXXU2CRED PDA in Telcel Carrier and when i tried to charge it turned off i always got moisture problem even with the charging port clean, then Telcel thanks to God released the G955FXXU2CRF7 the moisture problem was fixed but i got the same problem that you have.
So what i did was flash with the stock MXO G955FXXS4CRI7 and changed the CSC to MXO (to get OTA updates and Samsung Pay) and done, that fixed the problem. Be sure to Backup all your data.
I flashed with the CSC and wiped everything, do not use the HOME_CSC the first time, you will use it to match the CSC(along with the Comilation file) to the firmware that you are flashing, and be sure that in the HOME_CSC there is not the hidden.img or you will have the "hidden error" while flashing.
If you flash the HOME_CSC in the firstime and boot the phone it's for most sure that you will not lose any data but when you enter into settings you will see a lot of options with repeated descriptions, and you will have to flash all again and lose all your files and apps.