Small foliage like grass behaves almost like a random noise generator, as Lumen doesn’t simulate transmission through it.
In order to fix it, remove small foliage like grass from Lumen scene (disable visible in ray tracing or affects distance field lighting). If you use screen space traces then set shading model to thin foliage or subsurface, so that screen space traces skip it. When it’s set correctly it should be affected by r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal.SkipFoliageHits and r.Lumen.ScreenProbeGather.ShortRangeAO.ScreenSpace.FoliageOcclusionStrength.