Why are there shadows on objects?

Question

Unity

Hi all. There is a 3d model of the carpet in the game. When light is baked, incomprehensible artifacts appear in the form of shading.
You can see in the picture that black spots appear between the corners. Has anyone encountered this? I tried to change the HardAngle in the import settings, but to no avail.
Do you have any experience in troubleshooting these issues? What does it even look like?
Below are screenshots without a grid and with a grid