Samsung's reply on this issue :
This mail is in regard to your Samsung handset S8.
We would like to inform you that; this is not an issue with the device, this is the inbuilt feature of the handset, during
*Wireless charging or fast charging, the device may feel hotter and this is normal range of operation which does not affect the device’s lifespan/ performance.
If the battery temperature goes beyond specified limit, the charger may stop charging the device and start charging again once temperature goes down upto the normal temperature (e.g. 36ºC) and this is only a precautionary action for customer safety. This mode is called as*“Swelling Prevention Mode”.
Swelling prevention mode is triggered due to the high temperature. If the phone temperature rises beyond certain limit, the maximum charging voltage is limited to 4.15V which is equivalent to 70~80% (±*5%) of battery level. This operation is disabled and starts charging again once temperature goes down up to the normal temperature (e.g. 36ºC)