Snap to landscape does not work....Is my landscape missing collision? [4.18.3]

Hello guys, I’m currently working on an airport with curved terrain.
So I made this runway edge light mesh, and import it into UE4
I use a height map to create the terrain so the airport isn’t completely flat. So when I alt+drag, I can duplicate the edge lights in a line. And since the terrain is not flat, some of them sinks into the ground.

Problem: But when I lift it and press “End” key. It does not snap to the surface.(The surface has a decal on it)

I’ve checked the player collision and visible collision, and find my landscape is not in cyan color…Does it mean some stuff is broken?

More info: I tried to drag in a sphere, and enable physics simulation, and find this…
First I don’t know why it is given a horizontal initial speed.
After that, before it hit the landscape…It disappears.
I think it did go through the landscape and falls into the world.