Defrost system
A sheet of frost on the back wall
You notice ice creeping across the freezer wall and the fresh food side slowly getting warm. The defrost heater has failed, so frost never melts off the evaporator coil. Once that coil is sealed in ice, air stops moving between the compartments — the freezer still looks cold while the fridge climbs past 50°F.
Usually a heater, thermostat or control board. Same-visit repair in most cases.