Hello!

I can't find where this comes from or how to solve this :

This is what it looks like zoomed out, in Unity :

Here is my mesh in Blender:

In Blender, these artefacts are not showing up with the solid viewport shading nor the material preview one, but it can be seen in the rendered viewport shading when looking at a medium-shadowed side AND moving the camera.

When I'm still, it's barely visible;

But I took a screenshot while moving the camera, and it looks like this:

You can see more of how it reacts, with my gifs in the comments below.

What I have tried /more details about the "problematic" faces

I didn't see anything wrong with those faces:

• Deleting one of the problematic faces, creating another one (fill) don't remove the problem (artifacts are still there), even before setting back the custom normals as I want (see section below)

• I checked for non-manifold or broken topology, everything looks fine, no double vertices, no ngons, I tryed every [Mesh > Cleanup] menu options to learn them and see if one was adapted to solve my issue: none

• Tryed recalculate normals

• All my face orientations are correct

• The mesh has
  ᅟᅟ- no smoothing group
  ᅟᅟ- no modifier

• This is not related with the debug material or texture in the blender: the artifacts are showing up even if the cube have no material

More details if needed, about the normals of the mesh

I precise that I use split normals to have custom normals shading.
I drew this schematics if you want to better understand what's going on there (what I wanted and what I did for the mesh):

(this way I can have a "hard" shading on the upper border and a "flat" sahding on the lower border, as you can see on the Unity screenshot)