In object mode with solid viewport shading, some of the faces here are clearly a different shade of grey despite all being on the same plane:
There's no material associated with the object and anyway as it's the solid viewport view, materials don't come into play. It's not a normals issue:
With face orientation turned on everything looks as expected (all blue on one side and all red on the other).
I've popped out the sidebar and looked at each of the faces, in turn, to see if any had odd vertex or edge data but all looks fine.
You can find the
.blend file here.