Why does Lumen produce different initial lighting results when reopening an Unreal Engine project?

When I first import a Datasmith file into my UE5 scene, the result looks like Picture 1: the lighting, shadows, and AO are extremely rich and heavy, almost like a cinematic photograph. The scene has very clear depth information from occlusion, with foreground and background objects well-separated and ordered — it looks like an offline-rendered CG shot.

However, after closing and reopening the project, the thick, deep AO is frustratingly gone. Instead, I only get Lumen’s small-scale, weak AO. With the depth information lost, all models appear flat, messy, and the whole scene looks dull.

AI responses told me that Lumen automatically applies background optimizations after the initial load, and these changes are irreversible, making it impossible to fully restore the look I had right after importing. But I’m not sure if that’s accurate.

So I’m asking the community: how can I keep the scene’s lighting and occlusion permanently in that initial, just-imported state, even after closing and reopening the project?

if all those were separate meshes and were merged into 1 large mesh then this happens.

Not sure why, but yeah that is a huge visual downgrade. :face_with_crossed_out_eyes:

Thanks. The model has to be merged because there is an extremely large number of meshes.

However, when I first imported the already merged model, Unreal Engine did produce an astonishing visual result. It’s just that this effect was later downgraded by the system’s default settings.

Yes, I’ll see if increasing the distance field resolution can restore some sense of depth.

After forcing hardware ray tracing for Lumen, all issues were fixed.Although I lost around 9 frames, the visual improvement is more than worth it.