In this weather the thermostat sort of 'hovers' close to it's seat remaining closed more than usual, when it opens to the radiator the cooling effect is so huge at -10deg then the radiator becomes almost too efficient instantly cooling the water so it should be no surprise that weird things are happening.
Having said that, changing the stat and re-filling with new coolant is highly recommended.
Cars in my care:
2021 Kia Spottage 1.6 Pez Turbo Dual Clutch Gearbox Trickery
2013 Renner Twingo - donkey work