Shadows appear in the deformed part of the mesh.

Thank you for your help.
Export the ZBrush modeled object to Maya, insert the joint, and deform it.
If you export the deformed mesh to Unity, a shadow will appear in the deformed part as shown in the image.

The face normal is facing outward normally.
Both Shaders are standard shaders.
There is no light in the scene.
The same goes for albedo.

We are looking for the cause and solution of this problem.
I would appreciate it if you could let me know if anyone knows.

Thank you for your cooperation.