Did you get an OTA to update to Android Pie, Or did u sideload it using adb?
Even I have been experiencing heating issues during normal usage on wifi. I am on airplane mode most of the time. Also battery is draining a lot faster than Oreo. I have disabled all the enhancements provided by Nokia and Pie upgrades but still to no vail. I have reformatted my phone atleast 3-4 times since Pie update but no help. I am left entirely clueless as if I try to use the phone to its full capacity it wont even last me for a day.