I've tested out the issue you're having and can confirm this is a clear bug when trying to render nParticles using Maya Software render. I even tried using Legacy particles and had similar issues.
I am able to render the particles, (even the ones that say "s/w") using both Arnold and Hardware render. However, I did have several Maya crashes as I tweaked variables and did single test renders (not ipr render).
One thing I did notice was the default shaders for the volume cloud was not what they used to be. Instead of using the default particleCloud shader that has existed since Maya V1 back in 1998, it created a new npPointsVolume. A shader, by the way, that gives ZERO results when searching Maya's help docs online. From what I can tell, it is using nearly 100% of the settings in the nParticle Attr. options like life color, transparency, etc.
I also tried using the nParticles and assigning an Arnold Volume shader. This worked, and I was able to control some of the basic shader attributes, but for Clouds, the result was lacking. Blobby surface rendered very well in Arnold (CPU only) using any basic Arnold shader.
From the looks of it, since Maya's Software render is very much obsolete for good reasons, My advice would be to use Arnold or Hardware render and render in layers so you can render the rest of your scene in other ways.
I only use nParticles to create cheap water effects and convert to polygons so I can render in Arnold GPU.
Also, keep in mind that fluids and bifrost are Maya's preferred methods for most "fancy" particle effects. They are more aligned with modern render engines like Arnold and other third party renderers.
