Point Density Particles Appear as Lines

I’m using a point density node to represent particles as sparks in an explosion. However, after appending the various explosion-related objects to another scene and scaling down the parent (with appropriate adjustments to particle velocities), the point density particles are now appearing as very stretched-out vertically.

Here’s the scene: WeirdPointDensity.blend (1.5 MB)

Any ideas what could be causing this? Thanks!