HDRIBackdrop creates really noisy hair on metahuman


I just installed UE 5.6 and I noticed that HDRI backdrop are causing this strong noise on the Metahuman hair. I’m looking for a solution, if anyone has any idea.

Typing
r.HairStrands.SkyLighting.SampleCount 128
in the console seems to improve but it still not clean. I can see a pattern in the specular.
Does anyone knows how to actually improve the quality?