Do you mean the mesh or the high temperature?
As you know the HEAT solver uses finite element method. It shows the results on the mesh. If you have something that is not meshed, such as air, it will not be simulated numerically. Instead it is used for its given properties. You can change the min and max edge lengths to make sure the meshing is proper.
I believe the resulting temperature is too high to be correct since some material properties will change with such high temperature.
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