DIAGNOSIS This is an overheating protection shutdown. The TV's internal temperature sensor is triggering due to insufficient ventilation or a failing component.
FIX First, ensure ventilation. The TV requires 4 inches of clear space on all sides, especially the top and back. Remove any objects blocking vents. Second, disable the Eco mode timers. Go to Settings > General > AI Service > turn off 'AI Energy Saving'. Go to Settings > General > Eco Mode > set 'Auto Power Off' to Off.
IF THAT DIDN'T WORK Check the power supply board. Overheating can be caused by failing capacitors on the power supply board not regulating voltage correctly, causing the main board to overheat and shut down.
HEADS UP Do not place the TV above a fireplace or in direct sunlight. The 65NANO75UQA panel generates significant heat. A cooling fan placed near the top rear vents can be a diagnostic stopgap.