Outside temperature is not the only factor that determines how quickly a heat pump will freeze. Air humidity also plays a very important role.
Cold and humid weather
When the air is close to 0°C and has high relative humidity, the outdoor unit can form ice quickly.
Very low but dry temperature
In many cases, lower temperatures with dry air cause less icing than humid weather just above or below freezing.
Why this happens
Humidity condenses on the very cold heat exchanger and then freezes.
Practical assessment
The defrost frequency should be judged in conjunction with temperature, humidity, and the actual condition of the heat exchanger.