The heater changes the local conditions and may increase cooling demand. That timing is relevant even though it does not prove the appliance itself is fault-free.
What the observation tells you
A room-average thermometer can miss heat concentrated beside the cabinet. The model's required spacing from heat sources matters. Adding insulation directly to the freezer or covering its warm surfaces can interfere with heat rejection.
A useful next step
Record when the heater is used and where the freezer sits. Check the installation requirements and arrange suitable advice about placement. Continue to protect food if storage temperatures are unreliable.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.