I have a Nucleo-F446RE Eval Board with a STM32F446 RET6U. The internal temperature sensor shows valid values of about 34°C with constant cpu load.
With the same program code on a STM32F446 RET6 soldered on the target hardware the internal temperature sensor shows a value of about -200 °C.
Both µCs have a factory-calibrated temperature value in the corresponding register, which is used for the temperature calculation. At the Eval board and the target hardware all supply voltages are basically the same. Why is there an difference? At several identical hardware versions, the temperature sensor shows always this behavior.