I already opened a thread in the answerhub 3 days ago: https://answers.unrealengine.com/questions/133871/single-line-trace-does-not-always-hit-the-object-l.html
Haven’t received any answer yet 
I also created a new project which just contains a landscape and is tracing the mouse, there I have the same problem. And I verified the Engine, but still same problem.
If it would be a bug, wouldn’t more people complain about this? I guess doing a trace on a landscape is nothing really special…