I didn’t use shadows for directional lights but don’t you need to give the directional light a meaningful position?
The light itself may not need a position, but the camera that renders the depth map of the scene needs a point in space to render from.
If your terrain is above ‘zero level’ then the shadow camera will not see it, because it’s located at point (0,0,0), looking somewhat down (hpr=(10, -60, 10)) and has a near plane of 10.
Viewing the frustrum (sun.showFrustum()) is a good way to debug and fix shadows …but not always