Particles disappear as Camera view faces same direction as particle flow

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?