With Instanced Stereo enabled, CPU Niagara particles are not visible, GPU Niagara particles render in left eye only.
1. Create a new Niagara System + Emitter and place in level
2. Enable Instanced Stereo in Project Settings (Restart editor)
3. VR Preview in level with emitter
Result: CPUSim particles are invisible , GPUSim particles render only in left eye.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-61777 in the post.