Black stains on texture after making object static

Hi,

i stumbled upon a weird problem while working on my scene in Unity. I'm trying to create mountains. After importing my model from fbx and making it static, texture gets messed up and weird black stains appear.: After making an object non-static the render looks fine:

I've already tried flipping normals, recalculating them, regenerating lightmap UVs, basically every possible combination of mesh import settings. None of this helps. What helps is to make object non-static as i mentioned earlier or setting a 'Rendering Path' to 'Legacy Vertex Lit' in 'Player Settings'. Unfortunately none of these are viable solutions for me.

Do you have any insight on what might be causing this issue and how can it be resolved? I would be grateful for any advice.