I noticed when going through my levels of my TC today that my second level, a city at nighttime, appeared to be bright instead of the buildings fading out into the darkness below. I went there again and it was fine...I then started suspecting the software render may be rendering the wrong type of light and saw that was what was happening.
I made a test map that has one of each light and saw it was worse than I thought; Normal, additive, and subtractive lights are all rendered as normal dynamic lights in the software renderer!
Here's the test WAD.