The problem is these black lines appearing, tracing around a layer change in the terrain, and the ever-present missing patches of terrain where it renders as sky instead of terrain. To recreate this issue, you must generate terrain spanning far, with lots of elevation changes in it. This is clearly visible on the grass, but I believe it might apply to other materials aswell.
In the image attached, you can see the black lines tracing the step changes of the terrain.
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