If it is doing this consistently every single time and it's opening the headlight 100% each time, it sounds to me like something may be plumbed wrong.
Basically the dropping engine vacuum is "telling" this one headlight to open. Beacuse it's just one side, carefully compare the plumbing and actuator switches between sides.
Can you get some extra tubing and test each headlight independently of all the valves, etc? Basically just plumb each headlight temporarily to make sure that each headlight is working fine.
