By default you will only get “PSOPrecache: Unknown” if you haven’t turned on precaching validation.
“PSOPrecache: Unknown” in Insights.
You can turn on PSO validation tracking with r.PSOPrecache.Validation.
r.PSOPrecache.Validation
From PSO Precaching Documentation.
Setting up PSO Precaching & Bundled PSOs for Unreal Engine
Optimizing Rendering With PSO Caches in Unreal Engine