I encountered this when building thin walls. Roughly speaking, the light shone through the walls and spoiled all the pleasure from the mod. I would be very grateful for help.
Version game: 1.12.2
Version mod: 1.5.0
CreativeMD commented
This cannot be fixed sorry. The light calculations do not allow it. I doubt this will ever change. Better effects can be achieved via shaders. Sorry, but I cannot help you there.