I don’t believe this to be correct, I’ve seen dev’s create entire environments using just planes and they don’t have the light leaking issues… There are so many videos online of people just using planes and their stuff doesn’t have the light leak issue either. I have also made an entire new model as you say 3d… and it still does the same thing… Light leaking through… So its not the fact I’m using planes as it still does it with thick walls… Everything is connected correctly all corners are merged and the object would literally be water tight… No gaps anywhere.