Box select always selects skeletal meshes in ortho viewports, even when not touching the actor. Switching on Strict Box Selection fixes it as a workaround.
Create a blank level
Drag any skeletal mesh into the level
Box select in an ortho viewport
The mesh is always selected regardless of the location of the box
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-13890 in the post.