Then you measured the temperature wrong. Please explain how going
from -20 to -19 can remove frost. And, IIRC, freezers are designed to
operate at about 0 degrees, not -20 ( I assume you mean f).

No. Freezers should be nominally at around -18C. We don't use Fahrenheit in
this country, unless you listen to Radio 2.

The frost free doesn't heat the food compartment to above freezing. That can
stay at -18C. It just runs hot through the evaporator to remove water. Some
older designs didn't have the fan/separate evaporator, so did warm the food
compartment significantly during the defrost cycle. My parents had one like
this about 20 years ago.

Christian.