I did a dumb and abused transfer brightness to get the smooth gradient lighting in the affected areas:

I noticed that the areas that were blacking out aligned with the heights of the dummy sectors:

Removing the dummy sectors immediately resolves the problem in Vulkan. So, I'm going to find an alternate solution to get the desired lighting effects in these areas and not rely on transfer brightness for now. The issue only seems to appear if I use
- Code: Select all • Expand view
SetLineTexture(lineID, SIDE_BACK, TEXTURE_MIDDLE, "FLD00");
from a sector where I have all those transfer light sectors. So, this is something we can fix on our end by using another solution for the lighting. That's for the best anyways, this solution always felt really hacky.