The OutHit from a LineTraceByChannel against a landscape in Editor is not returning the Physical Material of that landscape. While PIE there is no issue. While using BoxTrace (even in Editor) there is no issue.
I think the issue comes from a call to ConvertQueryImpactHitImp (CollisionConversions.cpp) where a if statement when the collision shape is a Heightfield is not executed if the function call comes from the LineTrace but it is executed if called from BoxTrace. There are more details and screenshots in the UDN case answers.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-195318 in the post.