this is incorrect behavior, I think a lighter (at Epic) should be asked to consult on the expected behavior of lights in general.
Try placing a light (point or spot; doesn’t matter) with a IES light profile- there is no way to correctly orient the source length of the light. You will essentially end up with a light that points IES lighting in one direction, and the length point in another- obvious design flaw.
To reproduce; create a flourescent light tube/fixture; drop a light (spot or point), give it a IES profile, then finally make it long by increasing the light’s source length- you’ll immediately notice there is something very wrong with how this feature was implemented in UE4. Should be an easy fix by the engine team; just reorient the direction of the light’s source by 90 degrees.