1) Due to the way the light components and the turrets are setup in Barotrauma, you can't have a light source anywhere but the origin point of the sprite's turret. At extreme angles,the light source moves to the left or the right of the barrel.

2)Why does this happen? Because the turret is coded in a way where the integrated searchlight does not follow the gun's barrel. Not precisely, anyway. The more the barrel swings clockwise or counter-clockwise, the more the origin point of the searchlight becomes offset vertically.

3) Maybe it's the mod's fault? No, it happens even with the default vanilla turrets, it's just isn't as noticeable. Of course, it's less of a problem there, because the default turrets are relatively thin and are structured in the way that doesn't attrack attention to the searchlight. Even more importantly, in vanilla turrets the gun barrel sits precisely where the origin point starts. With this turret, the barrel is situated relatively high in the gun, so any issues quickly become noticeable.

I might look into a solution. Maybe there's some weird way to offset the gun and sorta kinda fix it, but for now it works the way it works.