Hi,
By the experience on 2 Note Pro: 5 and 7, it's normal.
It depends on the charger power.
1A Fresh
2A Warm
3A Hot QC3/4
For them, the charging power decrease to 700ma if too hot.
If the device was cool before plug may stay to 30/35, if in use would even rise to 42°
QC here is the Xiaomi's...