I’ve noticed that the lighting and shadows of spline meshes seems to be significantly different to the an identical static mesh.
I made the sequence of Spline Meshes shown in the first picture, ran a full production lighting pass, then merged all the actors to the single static mesh shown in the second picture and relit.
Note how the shadows are significantly different. Both the splinemesh components and the static mesh have their Mobility set to Static.
Since both variations are immovable meshes, I would expect them both to appear identical. As it is, it seems that SplineMeshes are not really suitable for creating level terrain, and will need to be merged down to static meshes before shipping the level.