when dragging an object either from content browser or an already existing one in the world the object does not surface snap to the terrain. however it does snap to all other static meshes. I am unsure if i hit a hotkey to disable terrain snapping or what could have happened.
This just happened to me in 4.18.2.
Its simple to fix.
Press ` to enter editor console and type recreatelandscapecollision
Press Enter and wait for the generation to complete.
This may take a while depending on the size and complexity of your terrain and hardware specs on your development machine.
Note GrassTool grass maps will build immediately after the collision has been finished.