The hair loss just behind the hairline is caused by reduced blood supply. The reduced blood supply is caused by the incision itself which cuts off the blood supply from the front and by the tension caused by the hairline lowering. The hair loss is at its maximum after about 1 month. Sprouting of blood vessels across the scar will create a new blood supply from the front and as tissues relax the tension will go down. This will ultimately result in a regrowth of the hair. To what extent and how long it will take varies per patient but in general most of the lost hair will be back after 6 months. If you want to do something about it you could use some Monoxidil lotion.