Commenting on your list of items:
Item 1: No, nothing that is resource intensive is running. It merely is plugged in and gets hot.
Item 2: The battery is indeed charging when plugged in however, none of my other 3 MS Surfaces gets this hot when plugged in and charging.
Item 3: There are not any background processes happening when the Surface gets hot! It simply gets hot when plugged in as stated.
Item 4: It is not set for High Performance. It is set for Balanced Performance.
Item 5: I am not using the Surface on a restricted surface. As stated the device gets hot simply being plugged in.
I will run the Surface Diagnostics however, I've done that before...
After running Surface Diagnostics, it showed NO issues.
The issue as stated is the Surface Pro 8 gets extremely hot when plugged in. What's the solution???