I have a simple rectangular cube mesh that renders correctly in Eevee but has strange shadow effects on the top and sides when I render in Cycles. I've confirmed there is no Custom Split Normals Data, the normals are facing outward, and there are no other objects in the scene that could be casting shadows. The top pic is with Eevee and the bottom is with Cycles. I've also attached the project folder. Any ideas?
The Origin of your object is problematically far away from your mesh; as Ron Jensen said, it's floating point precision.
Apply the location of the object, or perform Origin to geometry and you'll be fine.