The environment geo is getting in the way and the particle's wireframe outline is the same color as the environment. A current workaround it going to the Preview Scene Settings and turning off the environment or changing the backgrounds color.
This is only an issue with Niagara, Meshes and material's wireframes aren't the same color as the background color.
The wireframe is very difficult to visualize when utilizing the default settings
The wireframe is easy to see
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-195650 in the post.