There's several photos that clearly show the port side was often over painted, but the angle of the glazing often doesn't show the starboard
A couple more pictures do allow you to see both and have the painted port window and clear starboard side.
What I've not come across anywhere yet is a picture that definitively shows both port & starboard underside nose windows painted.
I suspect Airfix have designed it clear as would be expected, and then looked at the majority of pictures that show the port side only and made the incorrect assumption that if one is painted or covered then both would be, and tweaked the fighter version mould accordingly.
Yes it's a fairly easy fix, if you have fine polishing tools at hand (and not everyone will have), but it's a very easily avoided error on Airfix's part as they could so easily have just left it clear and told you to paint it anyway.