When it does so it's because you're setting the external NTC, which gets disconnected when removing the handle, it goes to 99.9°C to indicate that, will come back to normal when everything is properly connected.
Setting it to auto will do the same thing, but fallback to the internal sensor if the NTC is not detected when inserting the handle back, instead showing NTC error.
Edit:
Sorry, I messed up, I had forgot how it worked!
NTC ON: Tries the external NTC, fallbacks to internal stm32 sensor if not detected.
NTC OFF: Always uses internal stm32 sensor.
NTC Detect option: Same as NTC ON, but allows to use two NTC values, for example different handles using 10K and 100K NTCs, tries to guess which one is being used, will also fallback to internal stm32 sensor if none is giving reasonable temperatures.
This is only useful for very different NTCs, like 10K and 100K, will probably fail to properly detect the correct one if they're similar (Like 10 and 15K)
There isn't such switch to toggle between handle and pcb NTC (Unless you did so in the pcb), only the stm32 internal sensor and the external NTC, which you may place anywhere, but the best is in the handle as this is where the cold junction happens.