Inverted Lightgrid Issue

[KillPixel]

I've discovered some strange lightgrid behavior in which vertex lighting will appear to invert (when a model would normally have the highlight on the top and the shadow on the bottom, it will now have the shadow on the top and highlight on the bottom). I was eventually able to fix this by raising the floor in a hull several rooms away. The map is properly caulked/hulled, brushwork is clean and nothing is out of the ordinary.

Has anyone experienced this before or know of the cause? This is something I'd like to avoid in the future if I can.

EDIT: Turns out if any portion of the map is below 0 Z this will occur. Very strange.