Heat not functioning in pure mode

Hi. I have a 2016 XC 90 T8. The first winter that I had it, the heat worked fine. This winter, the heat only works if the "additional heater" setting is on. This setting eats through the battery very quickly so I don't like to use it if I don't have to. Even once the car is warm, if I turn the "additional heater" off at that point, the heat stops working and cold air blows from the vents. I have taken the car into the dealership 3 times. Today they finally said that this is how the heat is supposed to work in pure mode. This makes no sense to me. Can anyone help me sort out what's going on?