I unplugged the PCB connector for the heater completely for now. Turns out that it takes more than a week of freezing temperatures for the defrost water to block all of the drain holes and for ice to start building up the heat exchanger fins. It would probably happen faster if it was set to more than 8C of target temperature.

The long term plan is to add this ETI SMR-T timer relay (or any similar timer relay) between the PCB connector and the heating element to turn on the heater only at regular intervals. Hiding the relay under the top cover of the outside unit would allow to adjust the timer intervals easily.