While I was writing this question, I figured out the answer:
I have written a weapon that is supposed to drop a 9×9 grid of bombs on the target location. And it does that very nicely. However, the grid is always perfectly aligned to the map edges, exactly north-south and east-west.
Why does that happen? All of my habs tags are false. Is there some undocumented effect happening? For example, does h go undefined when v is 0 or 180?
That is indeed the case. When I try to Redirect upwards, Translate, then Redirect again, the original h direction is lost and presumably set to 0. Could this side effect be added into the documentation please?
After rewriting the weapon to lay out the grid at ground level first, then translate upwards in the final bomb itself, the effect occurs at the correct angle.
The original code (reference accessories removed, but easily imagined):