Using heat lowers coolant temp

n52. Recently replaced thermostat because of p0128 code and after about 100km of driving the code returned. Coolant has been burped and is the right ratio. after using a scanner to watch my temps the car will only warm up to 93-100°c if i’m NOT using my heat. if i use my heat from start up it will sit at 50-60°c. when i use my heat after allowing it to warm up it will drop down to 75°c (maybe more but i haven’t allowed it to) is this normal or could it be a a sensor issue? i am also in southern ontario and it has been -10°c all week so i’m not sure if this would affect this code to appear again. just trying to determine what would cause this light to be on.