i'm noticing this problem too.
are those planes overlapping?

quoting from the 3.1 release notes:

"Ray Tracing Precision
Many artifacts from rendering small, large and far away objects have been eliminated. There will always be object scales where numerical precision becomes a problem, but it's further out now.

There can still be artifacts with rendering overlapping geometry, in some cases more severe than before. Such overlapping geometry should be removed, or have a small distance added in between."

@Riccardo Fasoli (fasoric) Yes, the hair planes have 2 copies overlapping. And one of them the normal is flipped.
This is a game model. I think using this geometry hair technique is to achive the effect that the hair is perfect from most of the angle of the view.
I'm not sure if it will be a problem that geometry hair has intersection.

But I found the transparency depth calculation a bit confusing. As the project file I uploaded, there are 22 planes and a cube at middle. 10 of the planes stays between the camrea and the cube. If my understanding is correct, I need 10 transparency depth to avoid black issues. But the truth is 10 transparency depth is not enough. It still shows black shadows from the planes inside the cube.

And another question: Is there a plan that introduce a technique just like sprite in Redshift that has no limitation of the transparency depth. It can be used at leaves and geometry hair.

@Philipp Oeser (lichtwerk) Sorry, that's my misjudgement. I think maybe the reason that causes the geometry hair showing black artifacts is the algorithm of transparency depth. But I still don't understand why I need 15 or more ray depth to clear the artifacts even there are only 10 layers of transparent object.