First happened to me only once and i wasnt able to replicate it again. Background turned random textures and only restart of the game fixed it

Second one happends regularly. Whenever i compute shadows the dirt terrain turns dark and it stays like that.
Before calculating shadows:

After (i calculated it in every quality, problem was the same), fastest shadows resulted in the dark patches turning white

When you place a ground block, the terrain on the same block turns normal
Turns out accually my friend sees the same
Here are my game setings, you can try to replicate this

Edit: It happens also on another maps
Edit2: I put the topic in the wrong place whoops
Edit3: Update