Dark grey artefacts on a simple mesh I'm modelling a coat for a character, and I have those weird dark grey artefacts on some faces.



A few things :
As you can see, there's no overlapping faces, no triangles either, it has no modifier on it except for the mirror modifier.
I've already tried to :

Shade it smooth or flat,
Flip and Recalculate the Normals
Apply all transforms in Object Mode, and
Clean Up for Loose Edges/Vertices/Faces, and Merge By distance.
I've also deleted and rebuilt those faces but those artefacts remains.

Any idea ?
Here's a blend file : https://blend-exchange.giantcowfilms.com/b/ykzVrBss
Clear (or edit) your custom split normals data from the Object properties-&gt;Geometry data section.  These custom normals are causing this issue.