I am using Unreal Engine 5 and there is the same problem.
It seems that splines do not form a smooth surface for the phyisk when using splines (whether Simple Collision or Complex Collision).
Have tested different meshes here and there are always bounces when using a ball / sphere (not always), at the transitions of the contiguous meshes of a spline.
Only if you merge the spline there is no problem.
But since you can’t just merge landscape splines, that’s not an option for me.
Epic Games, please fix the problem.