4.6is strange coolant temp issue

[x5Alpine]

So I unlocked the cluster to display actual coolant temp on my 4.6. it has always been around 105c and gone down to 95c when gong up hill for a while as designed. However, this evening it hovered constantly around 90c but the second I switch the engine off it immediately reads 105c. does one coolant temp sensor work only when the engine is on (radiator hose sensor or the sensor on the thermostat). I would say that its the thermostat but that doesn't explain how the temp immediately changes from 90c to 105c after the engine is shut off.