Niagara particle stream disappears as camera angle changes

Something quite unanticipated is happening with my Niagara smoke exhaust affect. When viewed perpendicular to particle flow the effect looks fine. As I change the camera angle to be the same as the particle flow the particles gradually disappear. In the first image below the viewport is at a 45 degree angle to the particle stream and the particles are starting to disappear.

Not sure if this is a Niagara particle setting or something to do with my lighting (HDR + ExponentialHeightFog + Skylight).

Any Niagara experts that can help?