Just the usual purple-black squares. Using Forge 1.20.1 on the 9.1 version of Better Leaves.

Does this apply to all leaf piles, or only those based on modded leaf blocks?

It seems to specifically be the leaf piles Immersive Weathering adds based on other mods, such as Chipped, Derelict, Burnt, Quark and some LetsDo mods. The vanilla leaf piles work fine from IW work fine.

9.0 works fine, noticed this on piles from redwood forest after updating

This is a solo layer