GPU Scene reserved resources can break Niagara mesh preview rendering

When a Niagara emitter has multiple Mesh Renderers and each renderer draws a different subset of particles (via Renderer Visibility), all renderers are drawn using the color of particle index 0 in the Niagara Editor preview viewport. Particle transforms (positions) are correct; only the per-particle attributes read in the vertex factory (e.g. Particles.Color) are wrong.

Once the issue occurs, it persists until the editor is restarted. Switching Preview Scene Profiles, toggling Show Environment, or closing and reopening the Niagara asset editor does not fix it. The same Niagara System placed in a level renders correctly in the same editor session.

Work around:

• Set r.GPUScene.UseReservedResources=0 in DefaultEngine.ini
• Apply CL 54969077 to the 5.8 engine via cherry-pick.

A repro project provided by a licensee is available in additional info URL

1. Open the repro project in UE 5.8.3.
2. Open the included Niagara System (/Game/EffectTest/) and look at the preview viewport.
3. The two Mesh Renderers should be drawn in different colors, since their particles have different Particles.Color values, but in the preview viewport they are rendered in the same color.
4. Place the Niagara System in level editor and make sure it renders correctly.

Notes:

• In the repro project, the preview scene environment is disabled: the Preview Scene Profile loaded first at editor startup has Show Environment (bShowEnvironment) set to False. The issue only occurs under this condition.