Sorry for my english.

There are parts in the game where I have a kind of line in front of me that moves when I move forward, this causes the shadows to change. Shadows also flicker as I walk.

I got the same issues in some location. It looks as a big problem because you get this on the first island, so you think you will deal with this all game but only a few locations will have this issue. is not a big deal don't lose time like I did to find a solution that doesn't exist.

I believe the problems happens because the game will switch the LOD objects back and forth so the shadow will change based on the model resolution. For who doesn't know what this is, in short, 3D objects in game will have different models for the same object that will be swapped at runtime depending on the distance from the player. More distance then an object with less polygons will be used to optimize the performance. Trees sometimes are just on the edge of the swap distance, the model will switch from high to medium and back again, and the shadow will "jump" (low resolution object has less polygons and a different shadow)