Lighting behavior is unreliable for translucent lit materials, results are inconsistent depending on the editor camera's FOV and the editor viewport's width/height/aspect

When working on the level editor viewport or testing on PIE, the visual appearance of additive/translucent lit materials changes significantly (and abruptly) when varying the camera FOV or the viewport width/height/aspect. This makes the final visual result seen on screen unreliable, which represents a big issue for artists trying to tune the color, transparency and overall look of translucent lit meshes and, most importantly, niagara particles. It is also detrimental to artist teams working on a scene, since what each person sees depends on their current viewport size and camera FOV.

The issue does not happen with opaque materials, unlit materials, or translucent lit materials configured with "surface forward shading". It does not appear to depend on any other light or material translucency settings, and it still happens when post-process effects are disabled. Scalability settings might have some limited influence on the behavior, as a user reported the effect being stronger on the "medium" setting. The problem is visible on viewport view modes "lit", "lighting only" and "detail lighting", and is not visible on "unlit".

The change in visual appearance happens abruptly in several points while varying the level editor viewport camera FOV or the PIE viewport width/height/aspect. On the other hand, the appearance changes gradually when varying the level editor viewport width.

Please see attached video capture for for a better understanding of the problem and its severity.

1. Download, unzip and open the repro project.
1.1. The scene has 2 rows of 10 shapes.
1.2. The first row contains particle systems, the second contains static meshes.
1.3. Each of the 10 columns uses a different kind of material:
1.3.1. Translucent unlit
1.3.2. Opaque lit
1.3.3. Translucent lit
1.3.4. Additive lit
1.3.5. Translucent lit, volumetric non-directional
1.3.6. Translucent lit, volumetric directional
1.3.7. Translucent lit, volumetric per-vertex non-directional
1.3.8. Translucent lit, volumetric per-vertex directional
1.3.9. Translucent lit, surface translucency volume
1.3.10.Translucent lit, surface forward shading
2. In the level editor viewport, click the upper-left icon with three dashes and vary the editor camera's field-of-view.
3. PIE, then resize the viewport, first horizontaly then vertically.

--> On tests 2 and 3 above, note how the visual appearance of additive/translucent lit materials changes abruptly at certain points, except when using "surface forward shading" lighting mode. Also note that this odd behavior still happens if all post-process effects are disabled and/or the viewport's view mode is set to "lighting only". On the other hand, the behavior is not observed when the viewport's view mode is set to "unlit".

[UE-23848] Bug Status

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