The reference manual documentation for the onboard temperature sensor for STM32H745, RM0399, chapter 26.4.33 gives a formula for calculating temperature that is only valid when VREF is 3.3 volts, but lacks mention, foot note, etc of any kind that VREF is a factor. It does reference the datasheet which states that TS_CAL1 and TS_CAL2 we obtained at VDDA of 3.3 volts which helps but VDDA and VREF do not have to be equal and the significant term there is the VREF where those calibration values were recorded.
I suggest updating the reference manual "Reading the temperature" formula with the formula that cube generated code has in __HAL_ADC_CALC_TEMPERATURE() which has a term for VREF.
