Reported there before: UnvanquishedAssets/map-station15_src.dpkdir#3

I'm now sure it's an engine issue, and I now know the line causing this:

	lightColor /= clamp(dot(normalize(var_Normal), lightDir), 0.004, 1.0);

There:

We can find trace of this code in first commit of the repository:

It was removed in 0bb3550 (Add tiled light-index renderer) and re-added in 0949fda (Lightmapping fixes) by @gimhael.

I don't know what this code is for, it looks like there is less issues by not using this code than using it.