You seem to have exactly the same issue as we do.
Watch those shadows, they are a copy of a part of your map (just a little bit below the box), they are the same surroundings as your landscape shape has.
Our landscape shows the same pattern, like a shadowcopy of existing shapes.
This might help the team eventually to evaluate the source of this bug, im pretty sure its a UE5.6 thing.
Another interessting thing:
If you paint landscape layers in an area where those weird shadows are, while holding L-Click on the mouse, the shadows disappear.
As soon you release L-Click on the mouse, they reappear again.