Hi Galaxy, yes there are no redstone torches or anything that creates light. I've recorded a short clip with the problem, there is two cases. One with my building with the on/off light switch, and another with a tower with no light sources at all, but the structure is still lit up. It looks like it's a Quartz combined with Glass problem, but I'm not sure.
Only for the tower, i've used the /clone command for my skyscraper 🙂 But that could be the problem, do you know a way to fix it?