My vote on the sidewind as well. What happens is that, on downwind side, the wind will push the missile away from the laser beam. And since the missile is a beamrider it will go stupid, once the beam is lost. Upwind side is not a problem, since the wind will blow the missile into the beam.
The solution is to ensure the wind is blowing nose/tail direction before firing. You couöd also try disabling the auto-hover and let the helicopter drift with the wind.