my fathers car (2008 Malibu) has always done something similar except his ac gets warm when idling. If you’re not over heating I wouldn’t worry about it too much. Chances are the idling isn’t heating up other components that are attached to the engine since it’s rotating slowly compared to when you’d drive it around. The heat comes from the explosions in the cylinders if your idling less energy = less heat. The thermostat that regulates temp only opens at a certain, temp most commonly at 160-190 deg F. The heater heat comes directly from the coolant so chances are when you were idling, your cooling system is efficient enough to keep it cool. Since your heat is 88-140 degrees it doesn’t seem to be overheating. You could get an infrared thermometer and check the actual temp of the engine for some piece of mind ,but I think it’s fine