The tester said that this issue occured when he cast light as a spell. So yeah, more light sources.
I have gone into the same area and cast light on ALL my party members. I cannot replicate the issue, so I don't know if it's a video card thing, a graphics setting, or what.
I did look at one of my maps and there are a lot of light sources per tile. More than 6 in many cases. I just can't replicate the issue on my machine.