Does the unit have a back-up heat source? It may not because you are towards the south. But they are correct that when the unit is in defrost mode it is actually in A/C mode. It is drawing heat from your house at that point and transferring it to the outside unit to warm the coils to defrost them. In northern climates you would have a backup heat source that would be cycling or running ro temper the air to keep it semi-warm during this defrost cycle. The only way around this problem would be to install some kind of backup heat source like an electric heat strip or such.