I'm using Cycles in Blender 2.79
I have got a volume scatter material set up on a cube (0.2 density ) and a spot lamp (20000 intensity) above the volume (though I have the same problem if it's within the volume too) I'm getting a weird hard cut-off in the volumetric lighting and neither moving the lamp nor changing the intensity of the lamp changes where that hard cut-off is.
I figured out that the hard line is the edge of the volume out of camera clipping range. If I reduce the size of the volume then it renders correctly
I guess this is just how Blender works?