in 4.7.6 when you place splines on the landscape and assign Spline Meshes to it , it is drawn where it is supposed to as expected .
in 4.8.(2) when you when you place splines on the landscape and assign Spline Meshes to it , it is drawn way below your spline points as shown in the picture . Moreover if you try to move splines points which you put in the previous versions of the engine (and look ok) , it updates their spline mesh to go under the ground.
my projects consists of several landscape splines and all of them have spline meshes on them and i still need to put several more. so this bug exactly prevents me from upgrading .