The problem is gone now and it was quite a headache. I tried to correct the intensity of the sun when it was underneath but the blueprint didn’t work. Used this tutorial for that A new, community-hosted Unreal Engine Wiki - Announcements and Releases - Unreal Engine Forums
Although for some reasons I couldn’t connect the light source directly to the “set intensity” node [if you know why please let me know]
The solution of this problem was this - I already had a big bsp cube underneath the landscape, but now I stretched it so that it was wider than the skybox. There was another cube inside the first one with “subtractive” mode so that there was a big deepening inside. But anyway the light kept shining through the land. In the end what I did is just played the scene while being underneath the landscape - on the deepening of the cube. The first time that the light cycled - the whole place was lit and there were funky shadows from the land above, but then it changed and became dark. It seemed so that if the engine wasn’t fully aware of the existence of the cube before that.
Right now I don’t see any issues that were present before and I hope that it will stay that way. Hope this helps you in your work.