Today is a Blender Is Being Annoying day.

I have the cone of light from that streetlight as a Grease Pencil object with the fill layer set to Add at opacity 0.5, but it's just... making some of the objects behind it transparent. For some reason.

Somewhere there is probably a single checkbox I could activate to fix this problem, but I'm damned if I know where to look for it.
Turns out this is A Known Issue in which Grease Pencil doesn't like having objects with alpha transparency superimposed in front of them, so I need to find a workaround.
Got the result I needed in compositing in like, five minutes. 🏆🧠