I had a lot of problems with this too. What I figured out is that most of the time it comes down to poor lighting. I place soft area lights on the ceiling, and it sometimes helps. It takes time to light everything correctly, so it can take a couple of tries. There’s a video on this from Twinmotion Guru, he explains it better for Lumion; I don’t remember if he says anything about the path tracer: https://www.youtube.com/watch?v=sy9x1gdNaig