Ah ha! Apparently it’s an already reported bug. Unreal Engine Issues and Bug Tracker (UE-68641)
If you use the same start and end location for a sphere trace (which should technically work) it will not return a hit. So the workaround is just to add another vector in there with 1,1,1 and it will return a hit from Landscape. Weird that it works on static meshes and not the landscape.
