Hi, Landscape guy here.
The landscape spline control points and the resulting meshes are actually separate, and don’t have to be in the same level. This is controlled by the “Place Spline Meshes In Streaming Levels” property on the segments. If it’s on, the meshes are placed into whatever level is under the spline at that point (approximately). If you move components to a new level from underneath the splines, it won’t update the spline’s meshes into the new level until the spline is updated. The easiest way would be to toggle the “Place Spline Meshes In Streaming Levels” property off and on again.