I see a cheese poof on the tail, not the cockpit. Possible you simply didn't straight out kill the pilot. Does the 30mm fire all HE rounds, or a mix like the 20mm's? Also, no guarantee that a round on the cockpit necessarily equals a kill of the pilot, regardless.
And if there is in fact a bug in the 30mm, it will be shown in a statistical analysis of kills of the K4. I've asked Snailman, and you're more than welcome to do it yourself.
I will say that it will lend more credibility to your assertion than speculation from films, since we can't analyze the code ourselves.