OK, I have figured out what the problem here is: Nightvision increases your vision radius as though it were actually lighter outside. At some point, we overhauled the lighting system so that you no longer see the light cast by light sources if it's darker than the current light level outside, because it makes dawn/dusk look kinda ugly. Therefore, when you cast Nightvision, it's treated as though things are getting lighter around you, and you lose the outermost ring of your fire crystal's light, because it's darker than your current light level.
Probably safe to say that this affects people with the Nightvision trait as well, and they do not get as much use out of fire crystals. I'll have to think on how to fix this, will probably have to decouple the "ambient light level" from your "low-light vision" and use the higher of the two.
