That seems to be a bug with missile ships. I noticed the same thing with a hostile npr in my game. he has ships with quite a few missiles on a lot of his ships (fired at my colonies, and recovered tech data), and he clearly sees me as hostile. However, he will not fire at my ships despite them being slow and big and them being well in range (he does have active sensor lock on them). Although, he may see that as a useless effort because they do have stupid amounts of shields, armor, and pd so will simply save his ammo for a colony(?).