[UE5.8] Medium GI (Lumen Lite): False black blocks under moving objects & black ring on mid-distance

[1876141237]

Engine Version: UE 5.8
Quality Setting: Medium (sg.GlobalIlluminationQuality=1, Lumen Lite / Irradiance Final Gather)
Platform: Windows (packaged build & PIE)

Core issue:
Under Medium quality, multiple shadow and lighting artifacts appear. The editor viewport shows minor glitches, but PIE and packaged standalone builds make them significantly worse and fully reproducible. High/Epic quality is unaffected.

Symptoms:

1. Black ring / black patches on mid-distance trees when the aircraft does a sharp 180° turn. Flying forward restores the far distance, but a new black ring appears at mid-range.
2. Shadow update lag — indirect lighting takes multiple frames to catch up after fast rotation.
3. False shadow blobs on the ground under moving objects (a drifting car leaves irregular black blocks that stick to the terrain).