Sun/Moon casting through surface

Hi everybody!

I am currently working on building an isometric game in my style and have a fixed angle to the world object.
I kinda noticed upon adding a typical day-night cycle to my scene, that the moonlight was kinda outcasting the shadows of my sun and vice versa.
It is on all objects in my game (rendered in blender, saved as .fbx, imported into Unity), it is like the sun and moon do cast through the surface level and not really taking it into notice.

How can I fix this issue? Haven't found any solutions by now and there is no "3D" or "Weather" subjects on this forum either.


To make it more easy to understand:

Upper left corner: Moon is casting shadows while its still below negative level (still not over surface).
Lower left corner: Sun is still casting on objects while its below negative level (pass the surface, below ground).