Other objects are not masked.. only the sun.
I have noticed that on reflective (or is it refractive?) gunsights, the sun doesn't shine through the window, but it does shine around it. I've just always figured it was supposed to be that way. The PT Boat sight acts kind of like that, except it is square instead of circular... either case I don't believe it's supposed to be like that.
Also, I've been in PT boats hundreds of times in the past, and I've never noticed this before, so I'm assuming it's something with the latest updates, but perhaps I could be wrong.