App note from ROHM Semiconductors when measuring thermals along exposed pads on power devices. Link here (PDF)
When measuring the rear of the package with a thermocouple, to measure accurately, the thermocouple must be in close contact with the package. A thermal pad for heat dissipation is exposed on the rear side and this thermal pad is connected to the drain due to the design of the package. Since high voltage is applied to the drain, the same voltage is generated in the thermal pad. If you attach the thermocouple here and connect the data logger, a high voltage will also be applied to the measuring instrument. In this case, measurement will not be possible unless the measuring instrument’s rating is higher than the applied voltage.