Particle Emitters Break when using Particle Direction node in Material Editor

I have a material that relies on Particle Direction input nodes. With this node active stationary particles appear invisible. However once given any velocity input in Cascade or Niagara the particles become visible. The material seems to break when no directional vector is given, rather than just outputting 0. Even setting velocity to something like 0.0001 causes them to appear as normal. I’ve tried defaulting the Particle Direction node by adding 1 but the particles never appear if the Particle Direction node is hooked up, and no initial velocity is given.

I’m unsure if this is a bug or my lack of knowledge.