sounds like its weighing the washing and using its Ai washing feature of the machine to determine that 2 rinses will be enough for that particular wash , to save water and energy and total wash time.
If the water level is not actually at door level and higher I dont think you really will feel the glass door getting warm , washing machines use so little water these days that most of the time the water level does not make it past higher than the bottom of the door lip .
2 ways you can try see if the heater is working OK :
1st is if you have one of these plug in energy meters and you have easy access to the washing machine plug and monitor through the wash and see if the wattage goes up when it gets to the stage in the wash where the heater turns on .
the other way is, and I am presuming you have the Samsung smart washer with Wi-Fi ? - well if you have the Samsung 'Smart-things' phone app has a great little facility to check your machines components (including heater) to make sure they are working as they should:
