I don’t know why, but this is the way how DebugLine is implemented by UE developers.
From the beggining, startpoint of DebugLine = ViewPointLocation, so it’s just in front of camera. Go 1 px forward and it will be “X”.
DrawDebugHelpers.cpp says, that the main method is DrawLine, which is implemented due to FBatchedLine.
I think, that this X-form of vector is more comfortable for quick search of its start and end points with one sight.