Yeah, by default planes on translucent FOFs (both sloped and non-sloped) display as if they were at full brightness or something. The middle texture of the colormap linedef special allows you to turn on fog block-like lighting, where the distance-based lighting actually is applied so I'm aware. =V
So really this may be "intentional" though I'm not sure either why translucent FOFs do that by default in the first place... is it to do with water?