Out of curiosity, have you tried the PBP code you posted with negative temperature values?
sorry no, i never actually used pbp for these sensors after discovering the real potential of esp chips


if the upper word was set appropriately here maybe that would help , adc_t might need similar treatment but the calculation should be
then ok ,i think?

Code:
    T1.Byte0 = IIC_RX[0]                            ' Calibration data into Unsigned Word
       T1.Byte1 = IIC_RX[1]
    T2.Byte0 = IIC_RX[2]                            ' Calibration data into Signed Long
       T2.Byte1 = IIC_RX[3]
    if  IIC_RX[3].7 then 
      T2.Byte3=255
      T2.Byte4=255
    else
      T2.Byte3=0
      T2.Byte4=0
    endif
    T3.Byte0 = IIC_RX[4]                            ' Calibration data into Signed Long
       T3.Byte1 = IIC_RX[5]
    if  IIC_RX[5].7 then 
      T3.Byte3=255
     T3.Byte4=255
   else
      T3.Byte3=0
      T3.Byte4=0
   endif