Have you set the material to TwoSidedFoliage? I remember I saw a video on just this topic: Unreal Engine 5 How To Fix Foliage Flickering / Noise
There are two console commands you could try:
r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal 0
and if that doesnt help try messing with:
r.Lumen.ScreenProbeGather.Temporal.DistanceThreshold