Don't forget that the pressure on the upper surface of the wing is still a positive pressure, i.e. putting a downward pressure on the upper surface of the wing. It's just that the downward pressure is less than the ambient pressure.
You are right that this reduction in downward pressure on the upper surface of the wing will cause a temperature drop.
But don't forget the lower surface of the wing too. It will experience an increase in upward pressure above ambient and so will experience an increase in temperature. It's just that the amount of change in this upward pressure is less than the change in downward pressure on the upper surface.
Also there would be temperature rise due to surface friction.
The net result???
