“Auto Defrost” is not a frost-free function. These units have a heating element on the back wall which runs once a day to melt off any frost/ice build-up created from condensation/moisture (especially in the warmer months).
This will run off into a small drain to your drip tray, if the drain gets blocked with food, this will prevent draining and may cause a large amount of frost/ice to build. If this occurs, a "manual defrost" will need to be actioned, as the element will not generate enough heat to burn off a large amount of frost.
To "manually defrost" your fridge/freezer you will need to switch this off for 24 hours, we suggest popping some towels on the floor & in the fridge door to keep it ajar.