The splines do not deform the landscape when the path is made with a straight spline. When you move a spline point to bend the road it will deform the landscape, but the deformation accuracy seems to depend on how the path is bent.
This was reported and tested in 4.23.1 (CL-9631420). This was reproduced in 4.24.1 (CL-10757647) and Main 4.25 (CL-10870012)
Results: When a spline is straight, without moving any tangents, the spline doesn't deform the landscape
Expected: The spline should deform the landscape in any situation
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-86164 in the post.