This problem is due to the intersecting mesh with alpha textures. It affects only Cycles renderings. To solve it, increase the default value of Transparency max bounces in the rendering settings. A x3 value, from 8 to 24, should be sufficient. Note: at the moment, a Blender bug is affecting some hair: https://developer.blender.org/T96537 The only solution at the moment is to use Blender 3.0 instead of Blender 3.1.