Quick answer: Unreal Niagara mesh particles missing shadows in PSO-precompiled builds? Shadow PSO not enumerated - add to Bundled PSO list or disable shadow casting on the particle mesh.
First frame after spawn: particles cast no shadow. Subsequent frames: shadows. Cold PSO compile takes 2-3 frames.
Add to bundled PSO
Niagara System's bundled PSO list includes shadow variants. Build-time precompile covers them; runtime never compiles.
Or disable shadows
For VFX that don't need shadows, disable. Saves PSO precompile cost; removes the bug surface.
Use the PSO cache profiler
UE5 stats: stat psocache. Per-frame PSO compile count visible.
“PSO precompile is whitelisted. Whitelists must be exhaustive.”
If your game ships PSO precompile, validate at build that every used PSO is in the bundled list. The check is cheap; the bug is visible.