Not sure what's causing this, but normal maps lit at certain angles don't seem to like having transparent pixels. If you move the mouse to the corners (or beyond) in the example .cap, you'll see it pretty strongly. It seems to happen closer to the source at lower Z angles, so I could circumvent it by increasing the Z height depending on XY distance, which would actually be a nice 3D effect... but I'm still curious if this is a bug, something I've done wrong, or just inherent.
I guess I should also say that light positions can't be checked at runtime, otherwise it would be easier to investigate. In the end, there are ways around it, including just not using transparency; it's just weird and undesirable; if there's some solution, that would be awesome.