UNREAL UE 4.19 - Why Rebuild lights ruins shadows and creates squared artifacts? - image attached.

Hi, I wonder why “rebuild lighting only” ruins shadows, specially on large objects… i actually preffer my scene without rebuild… but i wonder why this happens?

I attached an image of my mesh before and after rebuild shadows (my mesh is massive btw), after press rebuild lights… squared artifacts appear in my shadows, what can i do now and what can i do to prevent this?